Speaking as someone who has just got a (small) PV system installed the reasons were primarily to try and safeguard against energy price rises by offsetting some of the consumed energy by energy we generate.
Of course I would be lying if I said the FIT wasnt appealing to me, but after 25 years then what? Well wth luck the system will have paid for itself and any maintenance costs also by then, but what of its efficiency?
I doubt the same system will be operating at anywhere near the output it currently its, so I will be looking to replace the panels again more than likely, or will it just be a case of "it generates enough to feed the house" and leaving the system as it is as its unlikely much will be exported by which stage I may well be happy with the thruppence ha'penny being paid by the Intergalactic Norweb Federation.
Of course Im talking purely from the perspective of someone with a small 2.5kWp system, those with much larger systems may well be generating a large surplus even 20 - 25 years hence.