Thank God Cno has said that! :rofl:
The main benefit for a floorstander in a 5.1 system is for music reproduction. The rest of it gets a little complicated, but I'll try and keep it simple.
Some people like big speakers. They like to use what they've bought to their fullest, so may set their crossover points (between the sub and speakers) as low as possible, or even use floorstanders as full range. The benefits of this is that because the crossover point is quite low, there's no crossover from speakers to sub in an important frequency range (midrange). This obviously has benefits. They'll sound fuller, and depending on the speakers capabilities, can give a more effortless and rounded sound than using smaller speakers.
Then there are those (like myself) that follow THX guidelines, which is a set of standards put down in order to achieve a more predictable results in almost any room (harder to achieve with larger speakers). The crossover point is industry set to 80Hz, and this is to relieve the main speakers of some hard work in its lower registers (good for budget packages), so it can concentrate on a smaller bandwidth and reproduce that without interference from lower notes, and therefore with greater accuracy. With all speakers only producing 80Hz and up, they all "singing from the same hymn sheet", so to speak, producing a more consistent soundfield. The sub is relied on to reproduce from 80Hz and below.
As you point out bigblue, (and something I advocate), there's a huge benefit to having exactly the same speakers all round, as with my current system, and pretty much every system I have had for the last 10-12 years. But, that's not always possible, hence why we have wide, squat centre speakers and small rear speakers. Probably 80% (token made up statistic) of homes have a typical centre speaker with smaller rear speakers, purely because of available space and lack of freedom to set their system up to its fullest capabilities.
There's positive and negative points to both types of setup, it's just down to working out what you want to achieve. The THX way isn't the best for music, but it's not impossible. The big speaker way isn't the best for movies, but that's not to say that a fantastic sound can't be achieved.