[EldrickOnIce]

Not sure if an 'expert' in gym battles, but it is the only reason I played prior to Gen 2 coming out, and am sitting in 17 gyms. (We are bullies - tear down our gym and we will burn all of yours to the ground). Anyway, I wouldn't call this absolute truth, but I think it's very good advice:
All that matters for Pokémon to put in gyms is high IV, which translates to high CP. Also note that high attack translates into highest CP. And BTW, your high 80's or 90% IV Pokémon are definitely better than mine. I'm good to go with 75%+ IV with high attack. I am near the top of several 3000+ CP gyms where 60 points is the difference between 3rd place and 10th place in the gym, so at some point those few IV% points will make all the difference.

I basically have 3 subsets of Pokémon:
- High CP to put on gyms.
- High CP with best attacks to bring down gyms.
- 1500-1800 CP Pokémon with good attacks to build prestige. These will be water, electric, grass, etc.
So your Dragonite with steel wing is a perfect Pokémon to put in a gym. Dragonites are really only good gym placeholders, imo. There are far better to use to battle with (higher HP).

Aegypticus has a great point though. For example, prior to one of the last CP adjustments, Arcanine was a great gym placeholder. It took a big CP hit when they adjusted - which made them pretty much useless. They have changed the gym rules a few times and screwed up my roster of Pokémon that were effective. So keep in mind that what is a good strategy and roster now may not be at some point in the future.

Edit to add: Your lower CP Dragonite will still max out at higher CP than a 100% Vaporeon, for example. Within reason.