By "pointless," you mean that I slaughtered every army ever that tried to assault one of mine? Sure, they climb them, but they can't shoot, benefit from charge bonuses, etc. A unit of yari ashigaru in spearwall can slaughter multiple units of samurai before succumbing to superior troops, as an example. Also, cover bonuses to shooting, allowing more time for gunners to shoot, yadda yadda. So many bonuses, adding up to a castle being way, way more better than "pointless."
On the flipside, there was original Shogun's only being able to assault through the main gate. That was a true slaughterfest, with singular units holding off entire armies while fire is poured into the enemy from either side, through the loopholes. I rarely assault castles because of the difficulty of succeeding with minimal casualties in S2; it was a fantasy to even dream of assaulting castles in the original.
