Advantages of External HD vs Enclosure?

Though it's been a while, last I checked a USB enclosure + HD were cheaper than an external HD of the same size.

Any advantage of one over the other?

USB enclosure tends to overheat if you put any big HD in it. (meaning 160+GB) Even if they have a complete alu casing. I don't know it those lil vents they put in some help, but I doubt it.