The biggest problem with cheap mics for this application is in the top octave or two - such mics are usuall not flat, more than a little directional, and (most important!) variable from sample to sample.
This is very important if you are a speaker maker developing a new tweeter.
For typical audiophile use, it's not important - you mostly want to see that things are reasonable flat through the midrange, and to move your speaker around the room modes, and maybe to check that the crossovers are phased correctly. Flat to 5kHz or above is just dandy for these purposes.