You will not decrease the life of the freezer compressor since i t will be running LESS often than as designed (higher temps). Whereas my situation, with a cold box attached to a dorm fridge will run the compressor MORE often (more volume to keep cool).
BTW, when I first installed my system I had 5 gallons of water dwn to 32 degrees at a fridge setting of 5 (6 being the coldest). Now I have halfbarrel of yuengling, a cornie of HB, and a 5 gallon carboy (for thermal mass) sitting at about 38 degrees, setting at 3 1/2 of 6. Also, I used my kill a watt to estimate that the compresor runs between 10-20 minutes per hour. I have no idea if that is too much but it does run somewhat more than my other, non coldboxed frigidaire. FWIW.
PS: I don't think there is a 'right' or 'wrong' way of doing this, and I would be the first one to tell you that my setup is not ideal. However it was the best idea for my particular situation because A. I got the fridge for 75 bucks scratch n dent, B. I wanted the ability to house multiple sizes and numbers of kegs, and C. I wanted to bud it directly into the bar.
If you haven't purchased a fridge yet, and are set in your ways to hold a 1/2 bbl, then a craigslist kegerator is probably your best bet, then keezer, then upright freezer, then dorm fridge with cold box, as far as bang for you buck goes. But you also have to look at the construction aspects and what works best for you.