, one of the authors of World of Warcraft Programming(First Edition):
Unless you are an addon developer, you generally don't need to concern yourself with addon memory usage. In and of itself, memory use says absolutely nothing about how an addon impacts WoW's performance. WoW itself uses an order of magnitude or more RAM than even the most memory-intensive UIs.
Data mods like Auctioneer will always show HUGE numbers compared to other addons but they don't do any processing whatsoever unless you're actively using them (scanning the AH). By the same token, many memory-efficient mods can use up an extraordinary amount of CPU time (damage/threat meters, for instance).
As a user, you should never judge an addon's worth by the amount of memory it uses.