"Regardless of which backend storage one uses for ZODB it is relatively transparent to usage."
RELATIVELY? Care to define that term? If it's not 100% transparent to usage, then the user's left with the same problem we have with the ACS. When we've discussed database abstraction within the context of OpenACS, our goal to hide SQL differences from users 100%. Of course, that goal might be impractical for us, too.
Your comment that Zope users don't have to create tables is well-taken. ACS users don't have to create objects...
Any reason for me to believe that creating an object in the persistent store is any easier than creating a table?
I will admit that there's an appeal to the notion that you only code in Python and don't need SQL. From my point of view, I don't see that as an advantage to the average user, though, since the goal is to provide a comprehensive toolkit so that users shouldn't have to program at all.
For instance, your users are working on Zcommerce tools. The ACS simply provides you a comprehensive e-commerce module off-the-shelf. Not programming at all is easier than programming, even if the programming environment leads to wet dreams...
