While the many points of this article are true...I would like to point out that an SLA provides a means to punish the service provider...in hopes of better behavior.

I run an IT department with a massive MPLS network from ATT. When a circuit goes down - it is true that I don't regain MY costs by enforcing the terms of the SLA and collecting the remuneration that is due. However, I do many times collect a significant amount of credits at times...and that causes regional managers at ATT to take notice and start making infrastructure improvements to improve actual up time...which is what I want - and then that allows me to actually pay them for their circuit - which is what they want.