Joe, The X-mods have been around for more than a few years. There have been many many, changes to their rules, mostly necessitated to keep up with cost effectiveness.
Example; the soft-touch rev limiter was cost effective under the older H.E.I. ignition rule, at least, until the MSD distibutors and spark boxes rivaled them in cost. Now, these are allowed.
So too, as the Hoosier tire supply has dwindled, that changed(except at UCS. Probaly due to their proximity to Aztec's supply of used IMCA Hoosier tires).
The $140 (retail) cost shock rule.
The Tri-Y header>outlawed.
The fuel additive>outlawed.
The list becomes longer the farther back in history you investigate.

Also. The Multiple Spark (over 20 degrees of crankshaft rotation, depending) that an MSD delivers is from idle too 3,000 RPM's. It allows/helps in burning the typically over-rich A/F ratio needed at lower RPM's to keep the mixture correct under loads at higher RPM's.
As the fuel level drops(under load) so to can the fuel ratio. Having multiple sparks in the lower RPM's aids in complete combustion under no-load.

Factory H.E.I. modules start breaking down at 5K RPM's and their aftermarket costs coupled with shaft flex etc.
It made sense to allow/buy a complete MSD system for just over $300 rather than building an H.E.I.
A wise rule change in my opinion. This rule change alone has probably saved more engines from detonation than the 9.5:1 compression (now 10:1) rule.

Talked to a Chrysler powered X-Mod few years back.
Because of the "stock" Hall effect distributors common in production vehicles it was necessary for his team to custom build a distributor.
It was verbage in the rules that necessitated that. Since the beginning of 2013 it has read "any distributor type ignition" allowed.

The management of these rules to date has been exquisitely handled. Let's hope no one (tracks) waffles and stops the upswing of this class.

With next years I.M.C.A. allowed Quik-change rearend, I'd expect to see a swing in that direction soon.