NUL is actually used quite often both in mIRC and in other programs/scripts/etc. Just because you haven't seen it doesn't mean it's not used often. You likely just don't use/need the kind of scripts that would use it, so haven't seen it for that reason. Besides, both speed and efficiency are valid reasons for using/supporting something. Obviously, a switch that does the same would be better, but that doesn't mean that using NUL is wrong or bad.
Before throwing broad generalizations out about how it's useless and not good to do and so on, why not try using it with /filter on a very large text file and compare your results. You'll find that there can be a significant difference in speed on large files (i.e. not just a few milliseconds).
Btw, just because you can name a file with a device's name doesn't mean that you should. So you should never need to try and output to a FILE called NUL.
