MaxB and WEBF
My understanding is that IDM is a useful tactical data link, and was found to be very useful by USAF F-16s who trialled it during Project Goldstrike. It's NOT JTIDS in any way, but it's a very good way of getting a nine line into the cockpit, or for giving a Link 16 originated RAP to the IDM user. It's a good tool for OS support of SF, too, I believe, but we'd best pass over that. And it's as cheap as chips.
With specific regard to IDM on Jag, I'd point out that only 1 of the 3 squadrons does recce, and that to be seriously useful for recce you need PRISM IDM and a digital recorder. (PRISM IDM = a PRISM card in the standard IDM toaster). That gives near-real time recce capability - very, very cheaply.
My understanding wrt JTIDS on SHar is that it was specifically cancelled due to the size constraint, presumably because at THAT time they were looking at THAT kit.
JN