This has come up on this newsgroup a few times at least.
A friend who I've been helping out over the last year who bought a house which needed lots of work doing has finally just had a new roof (stripped back to rafters, new insulated sarking, battens, and clay tiles), together with facias and gutters. Very nice job all round, but done by a roofer, not us, although I helped with the odd bit.
It's a terraced house, and where the roof runs on across the neighbour's house, the roofer couldn't move any tiles to straighten them up or even to replace broken ones near the boundary, because it's been sprayed underneath with foam and everything's stuck together rock solid. Neighbour's roof does look very tatty now that there's a brand new one joining onto it.
Apparently, neighbour has asked about having theirs redone too, and the cost will be much higher because of the foam treatment, and the difference is nearly twice what the foam treatment cost! Talking with the roofer afterwards, he said in the time it took to strip my friend's roof, he'd only be able to get about 4 rows of tiles off the foamed roof. It has to be done by smashing the tiles off, and then sawing along the rafter sides to separate the foam/battens from them, and then cleaning it off the rafter tops. The mess is unbelievable and needs lots more skips. If it's been on there for more than 5 years, you always find some of the rafters have rotted and need replacing too.
Anyway, thought that was a brilliant confirmation of what we've always said here when the question has arisen.
He gave me another example too. Old lady had a roof leak which she asked him to fix, as 3 other roofers had refused. Thought to himself, can't be that difficult, although it was in a valley where two parallel roofs met. Got up there, found that nothing could be moved or replaced and the whole lot was glued into a solid lump with foam. Had to go back and tell her he couldn't do anything without replacing the roof, so that made 4 roofers.