Best use for these national summons is to store them in your laboratory in form of gems, imho.
Only Zmey is worth considering, if you are rushed perhaps, and he doesnt require magical leadership anyway. And for a flying commander you can summon an Alconost for 8 astal gems; a holy-3 summoned commander is extremly good if you have undead-happy neighbours imho.
P.S.
There's nothing amusing about Gamayun and his slots because Starets has more slots and same paths (possibly even better)