I get the goal of keeping content and presentation separate but the fact that presentation is a consumer of content means that even with strict enforcement all this does is make life slightly better.

Any website above 'very simple' requires a ton of work to switch themes. When's the last time you had a complex site and needed to dial in a new theme and it wasn't a long, difficult process?

Furthermore, as long as the customer is well aware they are being 'locked in' to a vendor's theme platform...which means in the future they actually may have an easier transition to varying themes within that platform...then who cares?