I don't think we're giving Tsunade enough credit here.
The only reason the elders asked her to be Hokage was because they KNEW what she was capable of. They're not gonna let some random, no - good person come and be Hokage! & apart from just elemental jutsus, there is the question of decision making and patience; if they didn't think she could handle the stress and cope with the worries of Konoha (plus her own), they would never have recommended her!
Tsunade's main point is her healing abilities and her super - human strength; the WORST thing to do in any situation is to avoid healing people and waste her chakra on the enemy. She made a wise decision and will resolve to fighting Pein only when it comes to that.
Its a bit like chess; you don't move the king until he's at a 'Check Mate'. Similarly, Tsunade won't put herself forth until she has no other choice.