Muslims believe in a particular religion, they are not a nation one can declare war on. So I don't think comparisons with Wartime Germany are reasonable.
For sure recruiting folk you feel unsure about needs consideration. But I think any ban would cause more problems than it solved. Refusing suspected individuals is one thing, a blanket refusal to recruit is quite another.
In any case should an IS sympathiser wish to apply it is perfectly possible they could get through anyway.
Plus it is vital that any action against terrorising military forces is done with the active consent/action of those who might be confused as part of the same overall group. We don't need to allow the chance to pretend it is one group imposing on another, but clarify it is combating an evil force in alliance, together.
Overall, provided the risks are known and due diligence carried out during recruitment, then I think it is a good thing. It's not risk-proof, but nothing ever is.