Any point of trading for DB/DC Dragonite if Latios handles the role better?

Latios is a legendary and requires a special trade, while DB/DC Dragonite does not, but this does not matter if you are offering a regional (which also requires a special trade).

dam i always wanted a full legacy dnite but non of my friends have one! ugh

Just wait for the research task.

I think the question is - what else do you expect to get for a regional?

A good legacy pokemon is a pretty solid trade for a regional. Aside from sitting on the regional for a month to develop a higher friendship level to lower the stardust cost (but simultaneously lose negotiating value as you have fewer people at that friend level to make you offers...) - what pokemon could you get right now for it?

Shadow Claw Gengar is the best offer you'll get (likely). And that's ONLY if someone has multiple of them, really wants the regional you're offering, AND probably fears it featuring in research since Body Slam Snorlax is right now. I.e. I doubt you'd get a Shadow Claw Gengar - I wouldn't trade mine.

So next best thing you could get? Maybe a level 30-35 tier 2 pokemon? (and it will get IV rerolled...)

Yes, if PvP ever come and the battle system is similar to what we have now, he will be one of the best options in the non-legendary category.