What I would like to see with the character builder is a "user made content" or "input your own data" function so I can add my own personal setting options, or 3rd party material to the options presented in the currant character builder.
Oh goodness yes!! Yes, yes!!
*ahem* Sorry about that. Anyway, back to the original question, aside from the obvious "it's ain't gonna happen"*, there's also the fact that there are several 3.5 character builders already out there. I haven't even explored this very far, but on my machine I have installed RPG Explorer, PC Gen, and DM Genie, and I'm sure there's more out there. The nature of the OGL vs. the GSL means that competing companies can make (and have made) 3.5 character builders, while for 4e there's only WotC (or someone with an amazing copyright lawyer and deep pockets to waste on legal battles). With the possibility of user generated content and exporting, even non-OGC material can become widely available.
So it could draw more people to DDI, but 3.5 players who want a character builder can already find decent ones all over the place. If there is a lack of creating your own content (or importing from other publishers), then it's even more dead. Sure some people would go for it that would never try out (or have been dissatisfied with) all the other 3.5 character builders. But that group would probably be so minimal that I'd guess random month to month fluctuations in DDI subscriptions would have a larger impact.
It might attract more people, but why would wizards do that? They want ppl to switch to 4th edition. And given how successful the DDI is, I would not be surprised if ppl start playing 4th because of the DDI.
I'll admit that I was very skeptical when all of the DDI plans were announced, and was a vocal critic during the early failures. But given where DDI finally is, at the current price I am pleased and it is shifting one of my groups to consider 4e part time. The DDI has been a large part of that. The other large part is the dawning realization that 80% of the stuff that annoys us about 4e is easily ignored fluff rather than the rules system.
* And not just because WotC is trying to limit the 3.5 market rather than support it, but also 4e was designed to be consistent and a digital component was in mind all along. 3.5 is all over the place mechanically, especially if you throw in some of the later books like Bo9S and ToM, not mention some of the wacky feats and prestige classes out there.