If by T-Shirt you mean a cotton one, then the advantages are clear, if you sweat anything like I do at least. Close fitting wicking material will keep you cool as the sweat will evaporate easily off you while you're riding. Cloths that flap in the wind are generally uncomfortable for long rides too. I tend to wear stuff that is unflattering for someone of my size, but I don't care because it's most comfortable.

Rear jersey pockets, they're one of those "once you've tried them you can't go back" things. They're just the most convenient place to put stuff like Clif bars, cell phones for easy access when riding.

I guess it depends on what sort of riding you do. If you're going out for 10 miles, then a regular tech T-shirt is fine. If regularly ride > 1 hour, I think the benefits become obvious.

The costs obviously can be high, much like everything in cycling, but you can find decent jerseys out there for cheap. I've got ones I paid $15, and ones I paid $100 for.