[JohnM]

You can use a USB mic, but the same caveats apply as when using a laptop or desktop mic input: the frequency response may be optimised for voice use (and so roll off at LF and perhaps at HF also) and the signal/noise performance may be poor. Without something to compare the results against it is hard to know whether a mic input is useful or not, though on a USB mic the frequency response (or at least its extents) should be in the spec somewhere.

 

[JohnM]

Why can't i use, for example, an AKG perception 100, a MXL 2001 or overhead mics with the REW software instead of those pencil shaped measuring mic?

The main difference between a measurement mic and a studio mic is usually the polar response. Measurement mics are omnidirectional, which allows them to capture the whole soundfield and makes them (fairly) insensitive to orientation. Studio mics are typically directional to varying degrees (cardiod, commonly) which means measurements taken with them will be very sensitive to the direction the mic is pointed and the contributions of the room coming from other directions will be atttenuated.