Monte, with the upcoming release of the 6.5" coil, is the Detech 7" a "necessary" coil to have in the arsenal?I will let my personal bias show here by saying .. No, I don't think so.
Honestly, I don't know why White's didn't just use the small, round Eclipse search coil from the start with the release of the MX Sport.

Those 6½" Concentric search coils have a very proven record for both serious Relic Hunters and Avid Detectorists on many well respected models such as the XLT and XL Pro, the Classic series III SL, Classic ID and IDX Pro, and the MXT series, M-6 and MX-5. I never could understand, and still can't, why they would go to an after-market search coil maker to produce a 7" DD coil.
Is the 7" Detech coil a bad coil? No, not necessarily, and when I bought my MX-7 I also bough the 7" DD coil because I know what it could do in places I regularly hunt, but also because I wanted to do side-by-side in-the-field comparisons of the 7" DD and 6½" Concentric. There are some people out there who might be newer to this great sport and don't have a lot of experience with a Concentric designed coil and have only used DD coils on their detectors. Then, too, some believe a lot of the quoted statements I read on forums or in magazines that make wild claims about when to use a DD and what they are supposed to do better.
I have some makes and models in my personal arsenal that work with DD coils and do an excellent job in some very tough iron nail infested sites or places I hunt that have high mineralization and are plagued with hot rocks. But I also have some detectors that work well, and better, using a Concentric type search coil in the very same environments. So there will be folks who like the 7" DD better and many of us that favor the 6 ½" Concentric, and in the end it is just a personal judgment call. I'll also conclude by saying the standard 950 coil that comes with the MX-7 is also a very good coil for searching more open areas, but ... If I were going to own an MX-7 and ONLY ONE SEARCH COIL, it would be the 6½" Concentric, and I'd be very well equipped for almost any site challenge I might encounter.
Monte