In reply to Marmolata:
I agree. I pitched an inner-first tent the other night in heavy rain plus wind, which made it pretty much impossible to just drape the flysheet over while I assembled the poles and inner. It was hard to get the pegs in and it was dark too, which is obviously a bit slower, so by the time I had it all done the inner was literally soaked through. Luckily I had a blanket to sponge up the worst of the puddles inside.
It's not often as bad as that of course, and personally I tend to go camping when the forecast is OK anyway. I don't find inner-first to be a deal breaker if the tent is otherwise good, but it's impossible not to consider it a big drawback for use in wetter climates. Once in a while you will definitely be caught out.
I think it's a popular design because: 1. not everywhere is as wet as Britain, and 2. it is reputedly easier to make a taut and stable structure if the poles are underneath the fly rather than integrated into it.
For what it's worth I have never once, in 40+ years of camping, felt the need to pitch a tent either as just the inner or just the fly, so for me a tent that goes up all in a one-er is a lot better than one that can be pitched in various less useful configurations.