POTS isn't affected by power outages, and while your phone line might be taken out during a disaster or really bad weather, for the most part it's hardened against damage.
The 411 On 911
Even if your VoIP or cell phone is working, you may find yourself unable to call 911.
A POTS phone is designed so that the phone system connects you directly to an emergency operator who will instantly know where you are, and be able to hold the line open so that emergency workers can stay in touch. Unfortunately, these capabilities aren't as available for VoIP or cell phone users.
Many VoIP services, including Vonage, perhaps the best known, do not give customers access to 911 service unless the customers ask for it. Even then, the calls are usually routed to a call center which then passes it along to the offices where the emergency workers are located. But it doesn't go to the 911 line; it goes to the office line. Your life-or-death emergency will be competing for phone time with emergency workers' business calls and phoning out to order pizza. You could easily find yourself reporting your emergency to a voicemail box.
Of course, some providers do give you 911 access as a standard feature, but you may still have to specifically tell them where the VoIP device is located. And you still may not get the 911 operator directly.