Here's my story: DS7 (then 5.5) was IDd by his K teacher as GT and was tested by the school. Now, I knew he was bright, and I figured he was MG. So when they ID'd him, I didn't even bother to go in and look at his test scores. They just send a "He's IDd" letter--no scores. He had a good K year, and they handled the GTness issue without my having to advocate. So who needs scores, right?
Then when he had major trouble in an undifferentiated 1st grade class, I went to see his scores on the WJ-3 (achievement test) and nearly dropped my teeth. He was 1-2 standard deviations higher than I had expected (and I had expected GT!): technically PG, not the MG I had expected!

It made sense though, once I read up. He was operating 3-7 grades above level pretty much across the board. (The areas requiring speed were lower--only 1-ish grades above level.) That's not MG!
So now what?
We decided to have him tested with the SB-5, and on a bad test day (getting sick and short of 2-3 hours of sleep) with a test that didn't suit him (as we had thought it would), he didn't test as well as the very experienced GT tester thought he should. The psych recommended that we test again on the WISC in a few months. But should we, we dithered?
After the SB-5, I had a major GTness crisis. before I saw the WJ-3 scores, I had thought DS was middling smart. Then I did some more reading and thought that the test was right and I had been in GT denial thanks in part to being surrounded by lots of GT friends and family members. Then we got the SB-5, and it seemed to say that he was closer to what I had thought initially. So confusing! While I wanted the support of DYS if DS was really qualifying, I didn't want to chase the score.
So we went back and forth about whether to re-test or not, but ultimately decided that because the in-the-know psych had recommended it, we should.
We waited a few months and tested in town on the WISC with a different psych, and DS had a much better test day. Plenty of sleep, no illness budding. And surprise! His scores on that test left no doubt in the mind of the tester or in my mind that DS was qualified for DYS. I decided that the SB-V and my belief that he was MG (before I knew about LOGs) were the anomalous data points, not the WISC and the WJ-3.
Quite a journey! And, yes, I guess I accepted the "heads I win, tails you lose" aspect of the testing. But I think the point of testing is to try to find a context in which to place what a child can do. As long as you're critical of the results you get, as long as you remember that none of these tests are designed to test the tails, and as long as you recognize that any test is a snapshot of one moment in time, not a comprehensive judgement on your child--who will be different tomorrow!--then I think testing is a good idea.
From testing, we learned that our particular public school wasn't going to be a good fit, mainly because they would not make the necessary accomodations DS would require. We also learned that he's deep, but not fast. That was important because it led us to decide not to put him into the private GT school yet, as I fear that he would be unable to keep up with the speed of the assignments even as he was bored with level of complexity, though I hope that as he matures and works on speed, we can revisit that decision. It also affects that way that I teach him--I know now that he's not dawdling, it's just how he is, and we have to adjust for it and work on it as surely as if he had an LD.
Without the testing, we wouldn't have known these things, and his education would be far less productive and happy than it is right now. We'd *ALL* be a lot less happy! Testing was very useful to us.
My take: knowledge is power. Testing is one source of knowledge. Take it where you can get it!
FWIW...