This thread sort of touches on the "is a beater worth it? topic.
I've done this, a number of times. In my case, it was never worth it. By the time you add up the cost of the vehicle, and then everything that goes into it over the years like insurance, registration, battery, tires, misc maintenance (all of which you also need to keep up on the primary you aren't driving as much), you could have either treated your primary vehicle to overkill maintenance and then some, or you could have had a decent amount saved up for a replacement for your primary over time.
It's ok to like your car and try to keep it around, but it's made to be used, driven, replaced. Seems like, generally speaking, saving them is a futile effort.