It's not holy because it performs miracles, it's supposed to be holy because Joseph of Arimathea brought the original bit over as a staff and it grew when he stuck it in the ground. As Stonelifter implies, that kind of behaviour from trees isn't so unacceptable.
Yeah yeah it might never have happened. And that tree might not be a descendent of one of the 'real' one (it was planted in the 50s? presumably from a bit from one of the trees at the abbey? i don't know). But surely you can appreciate a bit of the cultural significance the thing has for the local people, it's part of the identity of the place, a tangible link with all the glastonbury legends (yes, mumbo jumbo) that contribute to what the town is.
YEah it's just a tree but have you no romance? I think you're being a bit mean :) But I think you're right, they should get planting another one.