It’s not impossible to see Ghoultree being cast for as little as 4 or 5 mana as the game progresses. The single Green mana cost means that you don’t have to be playing strictly mono-green to cast him. Many of the more powerful Green cards have multiple Green mana symbols for a reason. Having only one means that you can potentially cast him for just a single mana. That’s a lot of value.
This card is full of flavor, and it’s actually a good card. Kudos to the design team on this one. It never became any sort of competitive staple, and certainly didn’t help top 8 any Star City Opens or Pro Tours. But, it definitely helped win a Friday Night Magic tournament or two. Zombie decks were extremely powerful during the original Innistrad era and this guy saw play when Return to Ravnica came around. Just as an out-of-nowhere big guy it was hard not to find a way to jam him into a fun brew.
Obviously, Ghoultree is a good card in EDH, where the likelihood of him being cast for a single green mana is not only possible, but likely. Sapling of Colfenor smiles upon this guy as a strong Treefolk ally. Doran, the Siege Tower doesn’t mind having him around, either. Jarad, Golgari Lich Lord and Sidisi, Brood Tyrant love having a ton of creature cards in the graveyard, so why not get a 10/10 for one mana? There are definitely applications. Being a Zombie doesn’t hurt his cause, either.
Just having a 10/10 beater without trample isn’t the best thing in Commander. But, if Ghoultree is cast for next to no mana or cheated onto the battlefield some other way, he can become a Zombie Elephant in the room that someone needs to deal with.