You should pose the same question to the developers of those other add-ons that *requires* Compatibility to be on. FSX is installed by default with compatibility OFF. Compatibility mode is usually turned on, for those add-ons that don't support Vista/Win7 permissions model UAC. Our software supports it entirely, which is why it runs on FSX with its Compatibility correctly set to off, which is how Microsoft installs it.
However, many problems with such legacy add-ons that don't support permission with the UAC model (basically, they try to write into the C:\Program Data folder, which is not allowed), can be solved by either running FSX As Administrator, instead of using the Compatibility mode.
Another possible solution, you can change the permissions for the FSX folder (and sub-folders), to add your own user name to the list of users that can write to that folder (and sub-folders), this is probably a better solution than running as Administrator. You do this with the "Security" tab after having selected "Properties" for the main FSX folder.
Another possible solution, but requires reinstalling FSX, is to install into a folder outside C:\Program Files, such as C:\FSX, this will usually solve all problems with such non-UAC-compliant add-ons