Well, a thread like that, which is devoted for some reason to criticizing a certain kind of speaker, is going to attract a certain kind of post. If you look at the specified and measured performance of Maggies, you'll find that it covers much the same range as dynamics, with -3 dB points ranging from 25 Hz for the big 20.7's to 55 Hz for the little MMG's. Also, while planars of any kind won't play bass as loud as a large floor standing dynamic's wooferw, they'll often play it louder than a small one.
The reason I mention this is that whether you want a sub or not with Maggies is going to depend on the model you get, how loud you play, and the size of your room -- just as it would with dynamics. Some will want a sub with the largest Maggies, some use the MMG's without one. So in my experience you don't want to go running out getting a sub until you have them and know whether the bass is sufficient.
If you do end up getting a couple of subs (always better than one), you can get whatever kind of bass performance you want. Sealed subs are less likely to color the sound than the horribly bloated ported kind.
The sub can also, besides increasing bass slam, take a load off the Maggie and let it play louder. Again, not important for most, but for those who like to listen at exceedingly loud levels, or have a small model like the MMG.
I should add that I've heard the 3.7's and the bass was superbly natural. Like any speaker, they're tuned for an average room. If you want to use them in an unusually large room, you can add diaphragm area with Magnepan's DWM-1 woofers. This will make more difference to the sound than a sub will, because it fills in the midbass, which is more important to the ear than the deep bass. (Not that deep bass doesn't matter as well.)
If you want to combine the naturalism of planar bass with hard rock slam, you have to get an old pair of Tympanis, as Luckydog said. After all these years, they still have the best midbass of any loudspeaker ever made. It's the way I went, but as this thread suggests, most people are happy with conventional subs -- Tympanis really do take over a room. And even the Tympanis can benefit from a sub to cover the range from 14-30 Hz.