Slot-A was produced for backward compatibility, for the OEM companies (Gateway, Compaq, HP and so on). They weren't really intended for the open market.
AMD intends for the Socket-A version to be mass market product. So you're thinking, I should just get socket-a Mobo/CPU and I'm covered for future upgrades. Well, here's the kicker...Socket CPUs are LOCKED and unless you've got micro-soldering experience (like the guys at TomsHardware.com) there's NO overclocking them (not even with multiplier dip switches on Mobos).
So, if you want to be able to overclock...then go slot-a
If you want future upgradability, socket-a is the platform.
Lovely.