SLA's are pretty silly and useless, as a rule. :) They're really only worth caring about if they can get you out of some long term contract. They'll never be good enough to counteract the losses you'll suffer from downtime.
SLA's are not useless, they are just useless for commodity hosting.
If (say) the NYSE was negotiating for a host, they would make sure they had an SLA that gave massive damages if the host failed.
But for commodity hosting, the provider writes the contract. There is no way they are going to expose themselves to your losses other than a fixed refund for the time you lost. Not just because of the costs, but because the costs would be a PITA to calculate. If Twitter goes down and alienates its customers, how would you calculate the damage?
Even "SLAs with teeth" are pretty useless, speaking as someone who's been involved in negotiating them (on both sides). Like I said, the most useful thing you can get from an SLA is a contract out. Even the NYSE is never going to recoup anything close to their real costs from an outage.
You don't ensure uptime with SLAs, you pick good providers and go for redundancy.
Additionally such agreements (essentially "insurance" against downtime losses) are only as good as your willingness to enforce them. Likely you'll be going to court over anything significant, and paying $100/hr or more to your team of attorneys.
