That's good for modders and good for Microsoft.
This makes Microsoft look like a good guy, letting the mod community make a mod based of some past beloved franchise they don't actually own.
I'd bet Microsoft is doing this b/c they don't know what the hell to actually do w/ the IP and not sure how to resurrect the license. Microsoft probably wants to see if gamers have an interest STILL in this actual IP. What better way than to test the waters by having a mod-making community make a mod for Crysis?
If the mod turns out to be really good and really popular, maybe Microsoft will contract someone to do another official Mechwarrior IP-based PC game. Maybe even contract the modders to do the next official MW game.
As long as the mod turns out well and the mod is popular, looks like a win-win situation, to me...