If I have to do hardware raid, I'll definitely spec in a backup controller. Learnt this the hard way when my raid 5 controller died years after I first got it and I could no longer find a replacement.
For high budget projects, having the extra raid controller as insurance isn't a big deal. But for most budget setup and cost conscious clients, soft raid obviates that hardware dependency.
You forgot flaky hardware. Boards that lose their configuration from time to time and what not...I heard stories about Mylex for example...