Cat 5 is rated for 100 MB/s, Cat 5e is rated for 1000 MB/s. At Home Depot at least you can buy either one for about the same price. Why not go for the higher rated cabling then?
In the olden days people thought you'd never need more than a 10MB harddrive on a personal computer. Never know what the future may bring. For example, what if you could play a DVD movie on a computer that has a DVD CD player and watch it on one that doesn't. A 10 MB ethernet connection wouldn't cut it. A 100 MB probably would but if you've got other things going on on that connection, maybe not. If you had Gigabit ethernet cards.......no problem.
Eventually, we'll probably all be using Fiber channel but who on a home level wants to make that investment at todays prices?