I own an OT but am considering a Machinedrum or a Monomachine to add to my recent Elektron fetish. Since the MnM and the MD are both VA can the MnM be used for drum duty on par with the MD? I realize the Machinedrum can be bought with sampling built in, but since I have an OT, this isn’t a priority.
Yes & No depending on your needs.
In short, the Machinedrum is an outstanding 16 track secquencer with focus on percussion sounds. With a lot of tweaking it excels in a lot of other areas as well, especially the UW version.
The Mononmachine is a 6 track sequencer (+ 6 midi channel sequencert if that’s any benefit) focused on different sorts of synthesis. Of course those synthesizers are great for percussion sounds as well but be aware there is no instant gratification for this occupation.
Personally, I use both and wouldn’t want to be without one or the other 
No. They are complements per design.
The MnM can do percussion, but it is far more limited than the MD, and will take you a lot longer to program beats. The MD will blow your mind, if beats are what you want to make. Tons of ways to make sounds and easy to edit. Also the MD has 16 tracks vs. the 6 tracks of the MnM, and that speaks for itself.
I have both, and one cannot replace the other.
I wouldn’t call the MnM a MD substitute, I’d call it a synthesizer with a sequencer that you can use to make drum sounds with if that’s your desire.
I just bought a MnM over a MD not because I expect the MnM to be a drum machine, but because I didn’t need a dedicated drum machine. I have an OT I can use for that by feeding it sounds from a MnM, OP-1, and MicroBrute 
Depending upon how you use your OT, your needs might be different than mine.
That’s a good point. If you already have an OT, you may already have all the “drum machine” you need. I’ve read a lot of people use the OT as their drum machine to great success. I don’t own one, so I really can’t speak about what it’s like to program beats on an OT, but imagine it’s pretty easy.