I'm relatively new to Marriott Gold, and I got two free nights in the MegaBonus promo this year. I sympathize with the OP in that I have so far found the free Marriott nights impossible to use. It seems that whenever I have tried to use them, there are never any Category 4 hotels around. (I couldn't use them in Paris, and I won't be able to use them in San Juan or in Rio, either.)

I suppose I could use them for cheap Marriotts, but if I the hotel is cheap, I just pay for it to help me fulfill my Gold challenge. (I could have used a free night at the Renaissance in Bangkok, but at US$100, why bother?)

I'm just learning the ropes with Marriott, so I know I have much to learn. But it did come as a surprise to me that I haven't been able to use the free Marriott nights, especially considering how easy it was to use my free Hyatt and SPG nights at hotels which otherwise would have been very expensive ($500+).