I bought my share in a 172 about halfway into my instrument training.
Although I had access to good rental planes with good equipment, it was nice
to fly the same plane, week after week, with the same radios in the same
places, all the same quirks, etc. How many hours did it save me? Probably
not many but given the choice I'll always stick to one plane.

Depending on where you are and the quality of rentals available to you, you
may need to buy a plane to actually use the instrument rating.

-cwk.