What I'm trying to say is that ideally you shouldn't use a solution that is less user friendly and possibly infuriating for your future customers/users.
The primary defense against OCR is to make the segmentation attack hard -- pushing the characters together somewhat. With more tweaking we could probably get closer to a sweet spot of just enough overlap. Not even all of the characters would have to overlap to be effective.
There's no defense against that... Which makes captchas just a big irritating bag of fail.
Perhaps captchas make more sense in capital intensive industries with clear avenues for abuse. In the case of SpiderOak, we'd prefer to avoid making the free backup accounts an attractive prospect for warez distribution. YMMV.