I wouldn't buy a boat that was previously owned by a guide company. Guide companies know just when to get rid of a boat. If it still had long term value the company would not sell it.
I think this is dependent on the outfitter, not only based how well they took care of their fleet, but also how boat turnover plays into their business plan. If an outfitter can replace their fleet every few years and sell their boats with only a few seasons of service on them for a higher price, they may well be cutting deals on boats with years of life left. Sometimes cash is more important than boats.
Also, at the outfitter that I work at, we treat our boats with care. The entire fleet is scrubbed and treated with 303 multiple times a season. Some boats are kept outside, but only the top boat of the stack sees sun; I figure each boat sees sun for 20% of its not-on-river season. Chambers are bled after every trip. In the winter everything is stored inside office space, in a temperature/humidity controlled room. Honestly, the boats' winter home is better than most of the guides' summer quarters.
I'm sure some outfitters pass on worthless boats, but ours are decent.