I don't really buy either of your premises, that 1) point buy systems have one optimal build and thus no other build can be used or 2) D&D doesn't have optimal builds.

1. In point buy there very well may be abilities that are better than others at achieving a concept. There may be an optimal build for the concept. That in no way restricts you to using only one build. You can build a "magic thief" in many different ways.

2. D&D has optimal builds for concepts. There are discussion boards dedicated to the subject. Usually the optimal build involves "play a full caster because it's a full caster."

You can use several different builds to achieve a concept in either type of system, despite the fact that some builds will be better than others. So what's really the problem?