I don't know.. Tirion was probably the most neutral of all people. If it was either Jaina or Sylvanas, it would be a shitstorm of "lol Horde pwned Arthas" or "lol Alliance pwned Arthas". We fought on behalf of the Ashen Verdict, good 'n neutral. Jaina and Sylvanas had already had their hopes crushed during the Halls of Reflection encounter, I doubt they'd be in a good condition to face Arthas again in battle any time soon.
I personally wouldn't want to see every major figure showing up on the scene where it'll turn into an anime team attack. ... or they summon a Captain Planet.