I'm adding planar reflection support to my deferred renderer. At first I was going to create a set of GBuffers for the reflection map (matching dimensions), but this makes the reflection map very memory expensive. So I was thinking to just reuse my main screen gbuffer, but only render to a part of it (using viewport). Is there any reason not to do it this way?

Is there any reason not to do it this way?

No, reusing buffers is the way to go, just take care, that you don't need the content of the buffer in a later pass.

It seems very common for game engines to put their buffers into a "render target pool", so that different stages of your rendering pipeline can share resources.