I encounter the same issue often: Granted update/insert to an user but forgot to grant it on the related sequence. It's hard to understand that an user has write access on table but not on necessary sequences. I think the grant on tables should cascade to related sequences. What do you think?
Wouldn't it make more sense for the grant on the table to fail with an appropriate error message? That would solve your problem, and it wouldn't be making security assumptions. Cascading permissions seems like a recipe for trouble.
