They can be useful. It all depends on what materials you want to cut with them, which you've not specified...
If, for example, you just want to engrave things, cut relatively thin wood and you're not too bothered about how fast it is, then they would be fine. The main reason for this is that all the machines you mention use unsupported rails, which are inherently less stiff than supported rails. This reduces the rigidity of the machine substantially, hence increasing the hardness of the material you're cutting, or the speed at which it's being cut (which both increase the cutting force), make the tool deflect too much for the machine to remain accurate. Fortunately they're quite small machines, so the effect isn't so bad, but it's still big concern.