If you do not have worthwhile units to suppose the alliance they can hinder you rather than help. I would not add an alliance unless I had worthwhile units to support the army. Of course as soon as I got my BD and GL up to level 10 it was on. Also keep in mind you can only take your level x 5 to battle with you. So a level 50 player can take 250 alliance members or 750 units to battle. That said it does not make sense to have more allies than that.