I think you are confusing two independent concepts.
Full disk encryption protects a dead system, that is a non-running system.
Once the system is running, FDE is no longer relevant. The running system has the same password risks and requirements as any other system as FDE no longer protects it.
Yes all passwords should have a standard that enforces a high level of entropy. If not for any other reasons than these two things:
Contributing toward the randomness of the password hash via the source password, and secondly the principle of defense in depth. It is foolish to rely on single layers of defense against compromise. In fact I would argue that ...
There are a few reasons why you'd still want complex passwords, though, they're all a little situational.
Local access or theft of your machine while it's on (or in sleep mode) but locked. A bigger problem for laptops or workstations in open offices/public areas, especially if you don't disable normal sleep and go straight to hibernate.
Remote access via ...