Yes, multichannel measurements it's so good, but not cheap, and needed good audio interface and much same microphones.
MasterLufter, I agree and disagree.
Agree: more-than-one-channel is very powerful and widely applicable. And the interface is important, but not to the utmost. So here I tend to be more optimistic.
Firstly, going beyond N=2 inputs might indeed get costly, also when it comes to the interface. However, as most audio-grabbers are stereo, the case N=2 is well within reach (financial-wise). And, secondly, one also does not really need two close-to-identical (and/or well calibrated) microphones, as long as these two are reasonably smooth in their transfer characteristic (both amplitude and phase). Note that we are not interested in anything like absolute SPL's but just in relative values, i.e. ratios. So, to do the 2-channel measurement one needs in practice 2 runs, with in the second run the two microphones being interchanged position as good as one can! Take the average of the two outcomes, and that (to first order) compensates for the differences between the two microphones, thus providing the ratio between the two SPL's at the two locations at hand.
Conclusion: I like a 2-channel option in REW ......, even if it would be a non-free option!