The only thing SLAs do is tell you how much you will be compensated for downtime. If a fibre cut happens, you can end up with a 16 hour outage regardless of what the SLA guarantees. It all depends on the nature, location and extent of the damage. A couple of summers ago one of my wavelengths was down for more than 16 hours. 3 x 432 count cables had to be replaced through 3 manholes because a fibre seeking backhoe ignored the locates. Splicing high count cables takes a lot of time (even with ribbon fibre).
I've always considered Verizon's (and prior to that, WorldCom and/or MCI) 100% SLA to be pure marketing BS.
Anyone who's ever worked with telecoms knows it's simply unrealistic. Fibres will get cut, carrier's routers will need software updates etc. etc. etc.
I reckon the reason you pay a Verizon-tax is so they have spare cash floating around to pay you the inevitable SLA penalties.
Personally, I prefer dealing with carriers who have more real-world SLAs.