Casey01, Axiom always wanted to build a bookshelf 3-way that could rival a floor-stander. That however required a woofer that could provide more linear bass than the standard woofer. Otherwise, Axiom would have to use two standard woofers to get the same linearity as the HP driver and that wouldn't go over very well for a bookshelf. When the HP driver came along, so did the bookshelf. By that time, Axiom also had an improved mid-woofer, tweeter and cross-over (more refined Family of Curves) so it all came together very nicely into the M5HP.
The M5OW is ported so there is some air movement. The HP driver is the only way Axiom could have offered a 3-way OW that rivals the M5 bookshelf.
Axiom doesn't offer the HP driver for other on-walls likely because it considers that market is well-served by the linearity of the regular woofer - particularly given the twin use of that woofer.
