Given the current rate of errata, buying updated core books seems kind of pointless to me. Even more so because I pay for DDI every month, which I use for more or less every rules-related issue in the book. So, no, I would not pay for updated books.
Unfortunately, quite a few updates are never represented in the CB; or at least you're very unlikely to find them there. All the basic updates (like stacking auras+zones, limits to free action attacks, stealth+acrobatics+athletics+bluff+endurance+insight+perception+thievery changes, dominate+restrained rephrasings, the revamp of skill challenges, the rewritten arial combat rules, rewritten mounted combat rules, vertical teleports, inclined pushes/pulls, marking+unconsciousness, rewritten aid another, resistance+vulnerability changes for combined damage types, thp nonstacking even from distinct sources, changes to the reliable keyword, conjuration changes, etc.) - of which there are
quite a few - are not included in or unlikely to be discovered with the character builder.
Actually, Zaran's DM is tired of the errata being used to fix fundamental problems in the game, and to tweak the rules of the game. He has decided that having to have a printout of the errata that is bigger than the PHB is not worth the effort, and will do any fixes by house rule instead of wasting paper on the errata every couple months.
Also, even tho I put yes in the poll, it isnt really true. 4e needs a 4.5e pretty bad now. Just look at the differences between the PHB1 and PHB2, just look at the universal distain for daily magic items, just look at the differences between MM1, MM2 and MM3.
I am not going to play the game where we have to keep track of many small changes through the CB just to play this season's version of the game.
I think you could easily get away with only looking once or twice a year. Unfortuntately, there's no easy way to find out what's changed in that time - the update page only includes the overal summary and the last changes.
In any case, Wizards should
really avoid updating basic rules willy nilly as they do now. At least power updates are easy to find in the character builder, but the basics are really a pain. I've started a thread in the 4e errata board arguing that the last particularly bad such change (concerning free action attacks)
should be rolled back. It could be replaced by power-only updates and it's vague and more complicated than the original amongst other downsides.
The 4e errata have gotten completely out of hand. I'm pretty sure there's nobody in any group I play in that's really up to date anymore.