Although a matched pair is nice and keeps you from having some potential issues, I have often used unmatched pairs without any issues that I could attribute to them being unmatched.
On micing a single instrument that will be with others in a mix, it becomes less critical than say micing a orchestral performance with a single pair where the total performance of all instruments is captured with those 2 mics.
(assuming single instruments in a mix):
Mics are matched by frequency response, and by output strength.
On output strength.
If one is like 3 db softer than the other and your mic preamp has notched gain switches at 6 dbs you can never match the output of one to another.
On a close miced
acoustic guitar this would give you trouble,you would notice it immensely where as on a drum kit you may not notice it as much. Of course you can bump up the gain digitally on you audio apps on the low channel.
On freq response.
If they are close, you will probably never notice. It just becomes part of the character of what you are recording. If there is a massive difference it will sound like you EQ'd one channel and left the other side alone.... probably not good.
I lost my train of thought...... oh well....you get the idea.
For drums I don't think there is too much to worry about with better quality mics.
