I just got myself some Sidis which take both MTB and road cleats (ie. my old time Atacs and also Shimano/look style plastic cleats), and while I'm waiting for road cleats to arrive in the post I've been using my atacs on them. Without the tread on the sole, the tiny cleat is sketchy as all hell, especially clipping in at traffic lights. I wouldn't recommend this in any way. Get some mtb shoes if you want to use mtb cleats.
I just got myself some Sidis which take both MTB and road cleats (ie. my old time Atacs and also Shimano/look style plastic cleats), and while I'm waiting for road cleats to arrive in the post I've been using my atacs on them. Without the tread on the sole, the tiny cleat is sketchy as all hell, especially clipping in at traffic lights. I wouldn't recommend this in any way. Get some mtb shoes if you want to use mtb cleats.