Depends on variables such as your case, its ventillaton, your particular drive(s), ambient temp, etc.
A drive cooler usually isn't necessary these days, but they certainly don't hurt. Everybody knows heat kills electronics.
Whether you should spend for one depends on your particular variables and how much you value your data.
I think of it as an insurance policy: hopefully I'll never need it .. but if I do ..
For me, tt's worth a few bucks to help preclude the headaches of a dead drive.
I have 5 of those IBM 75GXPs (45GB) .. the ones that are failing on everybody.
Had mine over a year now .. haven't had a single one fail (knock on wood).