Developers (in consultancies) usually charge by the hour too, but thankfully we don't tend to get screwed over that often.

My GF works in advertising/design, and things are pretty much the same there; work on pitches for free, then get paid to do the actual work if you win the pitch (pitches happen more often than architectural competitions, and involve less work and less competition, and as such are less risky to do, but the basic principle of working for nothing to get a job is still there.)

In any case, lawyers, architects and prostitutes are definitely not the only professionals with clients :-)