This is really faulty logic. You don't put a Choice Scarf on slow things in order to make them fast, you put it on fast things in order to outspeed everything. Choice Scarf Hydreigon is horrible because, even when scarfed, it gets outsped by other scarfers that will ruin it's day (Terrakion, Keldeo, Salamence, Garchomp, Latios, etc.).
I think Hydreigon is viable, but you really know how to use it. with base speed of 98 you really have to have the choice scarf set (unless you want to out sped by a lot pokemon)
I love using him don't get me wrong but Latios or Latias is way better. But once again it is viable. Even with base 98 speed it special attack makes up for it with base 125 Special Attack.
OT: Is Hydreigon viable? Technically, yes; you can put Hydreigon on your team and, if your team is built well enough to support it, you'll see a decent return of investment. But is Hydreigon worth a team slot? That's a much trickier question, and I'm leaning towards no.
I've had plenty of experience with Hydreigon, and I've found his talents aren't in particular demand in this metagame. He's fast, but not fast enough, and Latios and Terrakion are faster. He's bulky, but not that bulky, and Garchomp and Dragonite are bulkier. He's a good mixed attacker, but so are Kyurem-B and Salamence, and Salamence has access to boosting moves and an arguably better typing (Hydreigon exchanges a weakness to Rock and 4X weakness to Ice for weaknesses to Bug and Fighting... worth it?). His Draco Meteros are like nukes, but so are Latios', and Latios is faster and not quite so weak to priority (but being weak to T-Tar sucks). All in all, Hydreigon is a quinessential jack-of-all-trades Dragon-type; he does everything at least decently, but there will always be Pokemon who can do one specific thing Hydreigon does better.
I think that one of the best tests of whether or not a Pokemon is worth a slot on your team is if, in the process of team-building, if you ever stop and say to yourself "You know what this team is crying out for? A Choice Banded Terrakion / physically defensive Hippowdon / etc." I've used Hydreigon before, but never because I thought he fulfilled a niche my team required, but because I just wanted to. As I said, you can use Hydreigon on your team and get a decent return of investment, but chances will always be good that you're better off using something else.
Also, Hydreigon does not look badass. He looks like a chubby little dinosaur with a flower on his head and two sock puppets for hands. I mean seriously he's just dopey looking >:V