Yes I have done this as well. In fact my old 240D still has the relay in place. It does indeed help but it really just patches the intrinsic problem. If the solenoid won't kick in you need a new solenoid or starter.

If the starter is a quality rebuilt unit without excessive end play, replacing only the solenoid will get you going again. You should not ASSUME that this is the problem, however, before doing a voltage drop test so that you know that you're not just throwing parts at the problem.

Good luck,