I am not so sure about the performance concern. This benchmark shows that several thousands of sparklines can be generated per second, so it seems that even with low-grade clients displaying hundreds of jobs the page rendering shouldn't suffer too much from it. I do agree that a server-side graphic generation doesn't seem a good idea.
Concerning the utility of such a column content, I would find it useful to view quickly (and without having to go in the job details view) :
- If the last build has failed, how many failed tests are responsible of that (sure, you can know this number concerning the last builds with the "Test results", but what about the previous ones ?)
- The failed tests trendline. Believe it or not, a lot of the projects I have seen on Jenkins were always "red" because there was this one test that failed all the time and whose resolution was constantly postponed. But then when another test came to fail also, we didn't see the difference. With a sparkline we could have known instantly that the job was failing, let's say, more than expected, and have a look at the list of failed tests.