For one thing, when things start to fail, you will know it cause you will need to stand on the brakes to stop the car (NOT kidding), and if this happens I would NOT drive that car till its fixed.
I forget what it is that if it fails, then you will have nothing at all, hence the need to fix what you have, or as some folks around here do, they dump the teves system and go to a non abs system. Plenty of info for that mod.
I won't be going that route as I'm keeping my car pretty much stock (worth more that way), but I have a couple of spare working units so its not a big deal for me.