It's not just the level ups, though. I use the Compendium/CB for everything 4E related - I look up powers and abilities when deciding what my actions will be, I look up conditions, modifiers, ranges, etc.
I'd also be making assumptions about all that stuff because I'm in two 'Living' 4E games that do use the errata - so in many ways I'd be keeping up with two different systems.
Every time I turn around I'll be posting an action only to find out it won't work like I thought it would and have to re-work it, which would have a serious impact on my level of enjoyment.
Again, I completely understand your position! One of Ruarc's abilities got nerfed before I even got to use it . . . frustrating indeed. But also one of the reasons I went with the DDI subscription rather than the books - DDI's cheaper, and I don't have to keep up with the errata myself. The downside is situations like this.
Referencing your recent XP comment (because it applies) - I don't hate 4E, I've very much enjoyed the games I'm in so far. But I much prefer Pathfinder, so that's where I put my limited gaming money and the limited time I have to actually learn rules etc. I'll play 4E games run by folks I like 'cause I want to game with them, but when it becomes a chore to keep up it's not fun anymore . . .