Wheel measurements

I've been looking through everyone's builds and noticed that no one rides the same size all around.  

1) Is this because the rear size would not fit on the front
2) If the rear specifications do fit on the front, what disadvantages are there, if any? (fitment, size limitations for all around fit, etc..)

I'm thinking it would be more cost effective to run all sizes the same for rotating tires, etc...
Thanks

It can indeed be more cost effective to run a "squaured" setup all around in terms of rotations and what not. i did this on a set of 18's i had once. I liked being able to keep it simple and have all 4 same tire sizes.
To answer your question (at least my personal answer as I cannot speak for anyone else's reasons) I went with a staggered set of wheels ( not the same front and rear specs) because i like the way it looks, the ability to get bigger lips and different offsets and because of the stance/ fitment i was looking to achieve needed to be different front and rear. So it comes down to your personal choice and if "saving" money is really up there on your priority list.
The way i see it, the whole trend of being on staggered wheels came about from the performance and racing aspect of things. more grip etc. and just caught on i guess...