I also have one mono Behritone, pretty cheap and awful sounding compared to nearfields.
I got it after reading 'Mixing Secrets for the Small Studio' (awesome book, everyone should read it) and, from what i understood there, the point is that this little cubes don't reproduce any low-end nor high-end, so you can concentrate on the mids -that is, where the money is, for most music.
This way, you can preview the translation your mix will have, if kick and bass can be heard on smaller speakers, if the snare-vocals relationship is fine, or if any other midrange-y instruments are masking each other.
For me, i constantly go back and forth between my main speakers and this, not just to check mono compatibility, but translation and midrange clarity. However, main processing is usually done in better monitoring, specially compression.
Apparently, there are also other advantages, as there is no crossover frequency, less room impact on playback, keeping your ears fresh and such.
Hope that helps!