The only time Pakistan may have joined an alliance was when Soviets were in Afghanistan as their presence next door was a threat to Pakistan it'self, recently Pakistan joined the effort in WOT as Pakistan was the one suffering most.
As for siding with Saudis, please recall the Gulf wars, Pakistan did send it's troops to the Kingdom but for only security duties, while many Muslim countries joined in to fight against Saddam Hussein's army, Pakistani troops remained well clear from participating. If need be, Pakistan will try and protect a brotherly country than join an alliance against it.