While I think the basis for the values of a summon should not be based on unusual circumstances, as is the Treelords, I have actually used them once or twice now.
Basically, I didn't have high nature mages, except for my pretender and I had better things to do with my pretender than summon tarrasque. Additionally, i had a site with conjuration bonus but none with Construction. So I summoned one Treelord and let him summon tarrasque after that (note that one of the treelords is actually 5N, though I didn't get that one).
Now, i could have forged items to do it but it would have cost a good bit more.
I don't think it qualifies it as a good summons at all...but I felt obliged to mention the one case where I found a use for it
- Kel