As so much of the distilled DCM will vaporize at room temp, such a pump need to be able to displace a lot more air per minute than one you use for distilling xylene, because the latter condenses considerably faster than DCM.
If you want to distill high-boiling compounds like certain freebases (tryptamines for example), you absolutely need a pump capable of pulling 1 mmHg or better, for general vacuum distillations 1-10 mmHg is pretty good too. However, if you want to use a roto-vap to recycle ether or DCM, there is no way you could use a stronger vacuum than 25-50 mmHg or so, possibly even less depending on the temperature of your cooling water and distillation rate. Therefore it is good to have a strong pump for product distillations, as well as one weaker which can take some abuse (water, corrosive fumes etc) for solvent evaporation and similar chores.