As I've said, it's an awkward one. Each case needs to be weighed up very very carefully before any work proceeds. The PC10 for example was an easy choice - at the time when it was modified originally I owned five of them - this was the non-runner, and my spare boards appeared to be faulty too. No further PC10s were harmed in the making of this production. The other four have since gone on to live with an XT-era PC collector in Christchurch, and I've obtained another example for myself in tidier condition - makes a nice contrasting shot when the stock as a rock classic PC10 is sitting next to the watercooled dualcore nightmare. Sort of like if you had a vintage '32 Ford sitting next to a '32 street rod.
A good modder never, EVER destroys something of major value, and anyone with half a brain about them would always do plenty of preliminary investigation first. Again, I speak for myself and the few other modders I know who work with old gear - there's always going to be some turkey elsewhere who will desecrate something truly rare (modified PET, anyone?)
The other problem is the quality of workmanship that comes with some mods. There's a huge difference between the likes of Iain Sharp's SE30 conversion (he used a 9" VGA monochrome CRT from a gameshow studio even) and the gung-ho hole-drilling type modifications that have been known to exist. Heck, I don't rate my own modifications that highly... they still need a lot of fine tuning to get them looking right.