Why depict TRSA on a map?
I think - and I may be wrong - that your question assumes that ATC services are the same in a TRSA and in class E in general. Or perhaps that TRSA Service and flight following give you exactly the same thing.
In fact, TRSA service is slightly different: it provides everything that Basic Radar Service does (e.g. safety alerts, sequencing), plus separation services for all participating aircraft. Usually, ATC doesn't provide any separation service to VFR traffic in class E.
The ATC Orders (JO 7110.65) sections 7-6 and 7-7 cover Basic Radar Service and Terminal Radar Service; you can read more details there. Since the ATC services available in a TRSA aren't the same as those available in general class E airspace, that seems to me like a valid reason to chart it.