I've experienced being turned down by taxi drivers when requesting short distances, as they have waited for some time to get in front of the line and don't want to "waste" it.
I recognize that it may be frowned upon, but is it ever illegal to take a taxi that isn't in front because they are less likely to turn you down? How about if you do it to get a nicer car?
Illegal? I doubt it, in most places. It's purely a courtesy thing, I suspect. Maybe some jurisdictions have rules or regulations, but I'd be astonished to see them enforced. You're more likely to get shouted at by a taxi marshal or the taxi driver for the taxi you join, as they don't like to see their fellow taxi drivers disadvantaged. They might well refuse to take you.
Don't forget that in most very busy places (big airports etc.) there is often a long 'off-site' line you aren't seeing, so even though they look like they've just joined the line, they may well have been waiting hours. You saving them 10 minutes from their wait isn't going to seem like a big deal to them, it'll just look like you're doing something odd.
Having said all that, in most places (for example, London) taxis aren't strictly allowed to refuse a ride, as long as you fulfill all the other standard criteria (e.g. requesting to travel within their "jurisdiction"). If they get disagreeable about it, insist. It might be polite to give a generous tip for a short distance, though.
