While allies are amazing in S2TW, they still backstab you a while after realm divide event, if you have vanilla, I play with Darth Mod so I don't get that problem.
But in general allies are good, I have an Oda campaign where my Tokugawa vassals control quite a bit of land, they're actually doing better than me since they're still able to recruit armies and stuff( I recently bore the brunt of a Kono sea invasion that has left me stagnant after putting so much into the defence.) I forsee that the tokugawa will basically carry me on their back to ultimate victory, I'm still Shogun though so my victory is not lessened by this aid.