IMO, GV is one of the more reliable providers. They may not be quite up there with Callcentric and Anveo, but they are surely better than most of the low-end guys. However, I do agree that support is lacking and it could be very difficult to get something fixed.
Whether a backup would help much, you'll have to decide. An issue affecting outbound is no big deal; you can just use your cell phone (and if the problem persisted, get something different). If an incoming issue affected only the VoIP path, you could likewise forward to your cell phone instead. And, if there were a problem with the DID or account such that inbound didn't work at all, a backup wouldn't help, because it would have a different number, which your contacts wouldn't be calling.
Personally, I'd still have a backup, with E911. Anveo is IMO the least expensive reliable one. At $0.99/mo. for a Value DID + $0.80/mo. for E911, your bill (if you don't make any calls) would be $1.79/mo.
If you have an emergency and are unable to speak, you may not be found, if you call 911 from your cell. Or, your might have a dead battery or be misplaced when you need it. Or, a guest, babysitter or workman in your home might grab the phone in an emergency and expect 911 to work.
A backup provider is useful for troubleshooting problems, and for working around them. Also, GV can't reach some numbers, e.g. those that depend on "early media" such as 800-426-7378 (IBM), or those in rural areas that are blocked because of high termination cost.