Pointless
Having little or no sense, use, or purpose.
Gen 4's my favorite generation and, frankly, I love pretty much every Pokemon introduced in this generation. Magmortar's cool with the big fuck-off arm cannons and being a mixed attacker with a specialization in Special moves, Staraptor and Garchomp's designs are slick and useful mons in any run of DPP, the Starters are all interesting and unique in their own right, and Rhyperior's a giant nuke...until you use Surf but don't sink my dreams and my poor big rock kaiju...
But take a stroll into the nice port city of Canalave and a hop on your mighty Gyarados, regal Empoleon or your poor poor Rhydon, you'll notice something odd in the water...
It's this fucking thing...
Finneon. Alright, well, it's quite adorable plus it learns decent to great moves like Water Pulse and Ice Beam but the rest of it's moves aren't exactly good...what stats does it have?
Oh...well, I'm not a stranger to raising Pokemon with bad stats. I mean, Duskull's aren't great either and it becomes a pretty alright tank (despite it's awful HP...) once fully evolved or the legendary Magikarp which becomes a monster only rivalled by Garchomp. Surely you shall be rewarded by Junuichi Masuda-san himself with a great Pokemon. I mean, Feebas from the generation prior to this becomes Milotic, a spectacular physically and specially defensive wall.
Ok, Lumineon? It's got a very nice design. So far so good. What is it's Learnset?
Ahh. Well, maybe it has good coverage through TM?
Err...Stats it is...?
...O-oh...Why the fuck does this thing exist? Just...why?
Even better question, why is it not available
earlier?! You can catch it at Route 205 which wouldn't be bad(?) If you NEEDED a Water type but there's two issues with that. For one, you need the Good Rod which you get it at Hearthome and, secondly, you can get 2 even BETTER Water types right there! Buizel and Shellos are available right there if you didn't pick Piplup. Hell, even Magikarp is a better choice because it at least evolves to Gyarados. Anything is a much better choice. Every single Water type in the Platinum Sinnohdex does whatever job you could give it so so much better.
Floatzel, Seaking, and Gyarados are your picks for Physical Attackers. I don't know what else to say here. They're pretty solid for a playthrough.
Want a Utility Pokemon?
Quagsire is surprisingly decent with Yawn, Rain Dance, Toxic, and (as average as it's stats are) it's typing is fantastic plus it has Water Absorb which allows it to switch in safely if, say, you're doing a Ground Mono.
Walls?
Tentacruel, Milotic, Empoleon are very solid choices as they specialize in Special Defense and have some good movepools plus Tent and Emp have great defensive typings so they may serve you well.
Pelipper could do good by you as a Physically Defensive mon. With a surprising 100 base Defense, decent 85 Special Attack and 70 Special Defense and moves like Surf, Hydro Pump, Tailwind, and even Roost by level up and by TM Shock Wave, U-Turn, Ice Beam, Blizzard and, of course, Toxic.
Tanks?
Whiscash and Vaporeon are pretty good. Massive HP stats on both but with some differences. Whiscash will be more on the Physical side (110 base HP with the rest being around the 70s with the exception of Speed at 60) and Vaporeon solidly on the Special side with it's 110 base Spec. Attack and 95 base Special Defense. Whiscash can learn Earthquake, Amnesia, Rest, Snore and Aqua Tail through level up with Stone Edge being the only move of note outside of the usual song and dance of "Surf and Waterfall). Vaporeon? You're not getting Hydro Pump nor Muddy Water lmao. Both are at level 71 and 78 respectively. Welcome to Gen 4. But don't worry, TMs are pretty good on Vaporeon as it can get Shadow Ball, Surf,
Want oddballs to use? Well, we have four!
Azumarill's pretty unique. it has a 100 base HP stat while having awful offensive stats (both at 50) and both defensive stats are, imo, mediocre (both 80) with Huge Power and Thick Fat as possible abilities, a unique Normal/Water typing and learns some neat TM moves like Iron Tail, Focus Punch, and Brick Break. Not to mention, it can learn Ice Punch through the Route 212 move tutor.
Mantine is another interesting pick as it is somewhat similar to Skarmory except it's biggest defensive stat is Special Defense at 140, a Water /Flying typing, and 80 base Special Attack. It can learn Psybeam, Confuse Ray, Signal Beam, and Hydro Pump at (Lv. 49, no less) through level up while the TM side of things aren't too great...but hey. It
can learn Surf and Ice Beam at least.
Golduck is, probably, my favorite of the three as it's stats are well-rounded but it has more of a specialization (pun not intended) in Special moves with Hydro Pump, Zen Headbutt and Aqua Jet being it's most notable moves through level up. TMs? Well, I'd say it's...quite good actually. Calm Mind, Ice Beam, Blizzard, Surf, Psychic, Brick Break, Iron Tail, Shadow Claw and some other moves. Pretty good coverage overall, imo.
Octillery has great attack stats (105 on both) and 75 on everything defensive including HP with an awful speed at base 45. It learns some good moves on level up alone: Gunk Shot, Ice Beam, Signal Beam and Hydro Pump. Octazooka is kinda good with it's 50% to lower Accuracy. You have better options for stabs but it's an option for some utility. TMs is where it truly shines with Flamethrower, Psychic, Energy Ball, Sludge Bomb, Charge Beam, Flash Cannon, along with the usual Ice Beam and Surf.
The only thing comparable in uselessness in battle is probably Bibarel...however, this is Sinnoh we're talking about.
The place that lives and breathes HMs so hard that GameFreak realized "We fucked up" and made HMs more bearable in HGSS and put them on the way side in the next generations. Bibarel learns Cut, Surf, Waterfall, Strength, Rock Smash, and Rock Climb but not Fly and Defog which brings us to 6 out of the 8 HMs. Lumineon pales in comparison with, as you may have noticed, only Surf, Waterfall and Defog.
Even then, you get Bidoof since the very start of your journey just in the tall grass near Twinleaf Town waiting for you and it evolves at level 15 with pretty decent stats for the early game and it learns Rollout (Lv. 13, even as a Bidoof), Headbutt (Lv. 18) and Hyper Fang (Lv. 23) through level up.
Finneon and Lumineon are just pointless and it's hard for me to say this without it sounding like I'm exaggerating. If Finneon/Lumineon were to be a Fighting Game character, the closest comparison I can make is probably Dan Hibiki from the Street Fighter series...before Street Fighter 4 but even Dan has a reason to exist unlike Lumineon. It really really sucks because I genuinely like it's design but it's just so useless and doesn't really have anything unique to it.