The problem: Customers paying for more than they 'use' pays for little things like support and sales. If a package is reduced to paying for space and bandwidth, then usually (everytime I've looked at these), it ends up being more expensive for the same package elsewhere.
Now, more expensive doesn't mean worse, as we all know, pricing is relative when it comes to web hosting. You can find the 'same' package at various hosts, and the pricing will range all over the place.
Any 'pick your own features' plan must include a base price for support, etc., then work from there. Don't forget to include that.
I don't think the model is bad, just that when people start adding it up, it ends up being as much, if not more, than a regular pre-made package, mostly because the host has already gone through this (or should have).