John in NM
Yes, they can be useful in routers.
Generally the two flute kind are most like router bits - cutting wood you need larger flutes (ie fewer flutes) because you cut wood far more aggressively than metal and you need the larger flutes to clear the chip volume from the heavy cut. Four flute end mills will burn more and dull faster as a result.
The other thing is you generally do not use them in a drill press. If your press's jacob's chuck is fit on a jacob's taper shaft, the side load from using an end mill to cut a slot or whatever can cause the chuck to come off it's shaft. It is a rare occurrence but spectacular and dangerous when it does happen. My Chinese mortiser had this happen, though not from side loading, probably from poor cleaning at original chuck installation.