Of course you don’t need it
To me the Monomachine is not that versatile. I guess that a Virus is much more versatile!
The Mono is my favorite synth. To me it’s the most immediate road to glitch. It has a sound signature (as any synth does), and I can’t emulate it. (Well to some extend it’s definitely possible to get similar results with other boxes, but it takes me more work, and I lose the fun side of the thing). I don’t use it everyday… For some sounds I definitely use other synths… But I do love it, and I find it a very fun synth.
There are many well known music made with it around. Some’s Autechre sounds are obvious examples. Midera has nice tracks with it too.
But I don’t state that the MnM is better than other synths… it’s just a matter of taste.