Interesting point about clients being confused with two different versions... Although it's a remote possibility, I could see it being confusing for someone who works with one vendor who swears by Classic and another who has migrated up to Next.

I also agree that this could be seen as inconsistent with their stance on making opinionated software. However, I put myself in their shoes and see only a couple of options - you either revamp Basecamp with a new design/user experience and risk a huge backlash from your customer base, or you build a brand new product and give people the choice. I'd go with the latter option.