Don't take this as definitive, but I seem to recall reading that taking tyres on and off also weakens the side wall, not a huge amount but if you are constantly swapping them about they will weaken.

Spare wheels or (with the cost of new wheels) spare shed of a bike for your main green laning tool. In this months TBM there was an article on Talon producing a range of budget wheels for trail bikes, no idea if any of the sizes that they are doing would fit straight onto the XT, but they may be worth a look, unless you are e-bay shopping (which would be way cheaper).