What does it make sense to bring back?

Here's something that's bothered me a bit about the current shuttle design.
You have a fixed size payload bay. And doors. Etc. Ok, that's fine if
you're a launch vehicle, but are you always one? And how much of what you
carry up is really valuable enough that it is cost effective to bring it
back??

It seems that once you're in orbit, all you really want to return is the
crew and the SSMEs. Most of the rest of the stuff, well, either leave it
at the space station for future use, or let it burn up. I.e. have a modular
launch stack composed of:

1. Engine and control module
2. external tank or similar cheap, non-reusable drop tanks.
3. boosters, as necessary
4. Cargo containers of various size

Crew, in this design, is essentially cargo: they can be sitting in their
return capsule inside a cargo container, or on top of the stack, or something.

Crew is ready for a return at any point in flight, more or less. Returning
the engine module with the crew module or separately is an interesting
question. Putting the crew module right next to the engine module seems like
a bad idea for a number of reasons; returning them together would require some
orbital dancing but it doesn't seem to horribly hard.

Basically, once you decide not to go 100% reusable, you have the ability to
decide certain components (tanks, structure, etc) just aren't cost effective
to reuse. Right?