My ds was tested by the EP at school, but only after they had already been working with him for a couple months on a weekly basis.
It was done because he was clearly very intelligent, but failing to achieve and also to help inform his assessment process. His results were complex and unexpected, so they had to do an indepth analysis, rather than just the standard/basic analysis.
In the end it highlighted processing speed and visual motor co-ordination as potential problems and these have since been borne out in other ways, through observations, assessing specific tasks, an OT assessment and a trip to the opticians.
In our case, the school has had the reports recommending extra time be allowed for processing, particularly in tests and exams, since January, but have failed to act on it.
Basically his verbal comprehension and perceptual reasoning results backed up the general feeling that he was cognitively highly capable, yet underperforming against his own potential. It helped us to identify possible problem areas that might be holding him back (ie the slow processing and visual motor issues) which we were then able to look into in greater detail.
We have been trying to get his visual problems assessed since then, but have struggled to find a window of time between other appointments and assessments. All we currently know is that he has better than 20/20 vision, but poor convergence and binocular vision, struggles with tracking from left to right and his visual motor co-ordination is poor. In addition, it has recently come to light that he's also bothered by some sensory stuff relating to his sight. I'm hoping we can get it all sorted before he goes into the next academic year.
As others have said though, the WISC IV has to be carried out by an experienced EP and with an open mind, as its really only a snapshot of how the child is performing on the day the test is carried out and under those particular conditions.
For us it was definitely worth doing, but only in addition to the observations and experience of the team that was working with him at the time, iyswim. I think if we had just had the test done on its own, the results wouldn't have been as meaningful.