You're right that ion thruster research is not necessary to enable trips to Mars, but it would make sense to try to reduce time in transit and requirements for launch mass, as well as enable more frequent trips to Mars. Ion (or EM drive) propulsion research could (reasonably) be regarded as tangential, but I would see it as forward progress which will help us to reach and explore Mars.

Failure to perfect high-efficiency propulsion should not be a barrier to Mars travel, but use of these systems would definitely be helpful.