It is much less of a problem then it used to be, but cell phones haven't reached 100% parity with landlines.
Consider:
1. Cell phones require reception. At my house, cell reception is universally awful for all providers. I may not be able to even make a call to 911 on a cell phone and, if I do, I am not sure I can trust the reliability of e911. Work arounds include in-home Wi-Fi and femtocells, but...
2. Cell phones require power. In a prolonged power outage (or in the case of my daughter, even a five minute one), I can't gaurantee there will even be a working cell phone on the premises. Wi-Fi won't work, and the femtocell won't work, either.
3. Cell phones get misplaced. In an emergency will I even remember where in the house I set it down?
That's not to say that cell phones don't also offer many advantages in an emergency. They do. Cell phones are usually available and usually close to a person. They can be turned on and hidden but still allow 911 operators to hear what is going on. There is usually about one cell phone per person, etc., etc.
But, at my house at least, I can't completely forgo a landline (although I have at other locations in the past).