What kind of control? Nearly any notion of control it seems you would want would be even more important for MTB given the crazy trail moves people there are doing, but the trend there is to flats (or so it appears to me with my limited attention on the MTB world).

There is something really nice about that locked-in feeling of a stiff bike stiff shoes and clipless pedals but not sure it translates to much, unless you are standing up pushing max watts. On a fixed-gear I can also see why the control could be better with clipless, it is a very delicate dance to keep an even momentum going when on an uphill doing 30rpm with a huge amount of force on the pedals. At higher rpm much less so unless also at very high force as in a sprint.

Gotta go find that old pair of flats in the garage.. if nothing else it will be fun to fiddle with after 25 years clipped in all the time.