Makes it wayy easier to update them all, compared to browsing curse.com etc. to find my adds.
Also, there are more updaters than just WowAceUpdater. I use WUU (also found on this forum), and it works really nice with curse and wowi as well as files.wowace.com. No need to limit yourself to only Ace addons.
Also, you may not know this, but not all addons that you get with WAU are Ace addons. Pitbull uses another framework (Rock). And there are some Ace addons that you cannot get through WAU, like SLDataText or PhanxChat. You can find those on wowi.
Only grabbing addons from this site, simply because it is convenient, means that you are really limiting yourself. There may be nice alternatives to popular mods here, but not for everything. There is stuff like Ogri'Lazy, oGlow, Fane, Butsu, Vendorbait, teksLoot and many more that you cannot get here, and where I simply haven't found any Ace mod that can compare. (personal taste ofc, I do not want to imply that XLoot sucks) Most of those are really small, and only get updated like twice a year, so even if you want to use WAU, it is not really a problem to go and check for updates after a major patch. Or even better: to turn on notification about updates on wowi or curse.