To my way of thinking the Exeter was completely intact and fully able to be redeployed. The ship could be held in stasis until such time it had been "scrubbed", thereafter it could be updated accordingly.
As for the Excalibur, is there not logically a modular aspect to the connie? At best it could be repaired and patched-up, at worst scavenged for useable components, sections, etc, for re-use.
I'd imagine a connie takes years to assemble, even in the TOS era, so it makes sense, if not due to budget, than for economy of time and manpower, to recycle, re-use and redeploy where possible.