I've mentioned Willow's alignment before in other threads, but I rather not anymore. Makes it funner if people think she's evil, good or neutral.  

But getting back on track, I don't think it takes an evil character to summon a Treant to help them fight fire giants. Fur burns easily too in a lot of cases, as do feathers. Point is, these creatures, along with the PC, usually have spells to help them resist or endure through the pains of fire. I've never seen anyone say anything about a panther, wolf or hawk in the spires. Are Treants somehow better or more important? I would think not. They're enduring the pain of battle along with your animals, elementals and PCs. And frankly, that's kind of the point, isn't it?