Could it depend on where the MDG is located?
If the MDG is saved within the same model's Resources then maybe relaunching EA is not necessary, but with an MDG saved to the local disk I guess the relaunch is needed so that EA goes to the disk (/server) and brings in the MDG definition from the xml again.
Just a guess based on some thing I've been doing recently.
In any case, I'm with the rest of you - "Troubleshooting step 1 is to turn it off and on".
Even with MDG's in the model you need to restart EA for some things.
Geert