SLAs can be very misleading. Assuming that downtime usually occurs after changes/updates, they can push those to Friday evenings and have most downtimes occur over the weekend (not nice for their engineers though). Most clients won't care if they're down for a few minutes on a Sunday night. An outage monday morning is much worse. Therefore SLAs often don't really reflect client needs, which is having a 100% uptime for 40-60 hours a week.

I don't think that shows the futility of SLAs. If their actual uptime was 99.991%, they'd have met their SLA but still paid a fortune in credits for the 0.009% downtime that did occur.