The point is that there is an awful lot more going on inside SQL Server than is exposed in execution plans. Hopefully some of the details discussed in this rather long article will be interesting or even useful to some people.
It is good to have expectations of performance, and to know what plan shapes and properties are generally beneficial. That sort of experience and knowledge will serve you well for 99% or more of the queries you will ever be asked to tune. Sometimes, though, it is good try something a little weird or unusual just to see what happens, and to validate those expectations.
Optimizing update queries seems trivial at first, but as Paul shows, we have a few more tools at our disposal than is apparent at first glance.