[Al Skierkiewicz]

Sorry,
I thought Phil was your name. Sorry. There is no mystery to the reason behind this rule. It is in the first paragraph of Section 4...

In addition, another intent of these rules is to have all energy sources and active actuation systems on the ROBOT (e.g. batteries, compressors, motors, servos, cylinders, and their controllers) drawn from a well-defined set of options. This is to ensure that all Teams have access to the same actuation resources, and to ensure that the Inspectors are able to accurately assess the legality of a given part.

Emphasis mine. Think about this rule as being as hard and fast as the robot battery, the PD, the legal motor list and cRio. The penalties show how serious the GDC is about enforcement. For instance...

G05
When placed on the FIELD, each ROBOT must be:
A. in compliance with all ROBOT rules (i.e. have passed Inspection),
If it is not a quick remedy: the ROBOT will be DISABLED and must be re-Inspected.

G03
ROBOTS whose operation or design is unsafe are not permitted.
Violation: FOUL & DISABLED. If the issue is due to design: Re-Inspection.

T06
A TEAM is only allowed to participate in a MATCH and receive Qualification Points if their ROBOT has passed
Inspection. If it is discovered after the start of the MATCH that a ROBOT did not pass Inspection, the entire ALLIANCE will receive a RED CARD for that MATCH.

T08
At the time of Inspection, the ROBOT must be presented with all MECHANISMS (including all COMPONENTS of each MECHANISM), configurations...

To use a second compressor of any type means that the robot has not passed inspection. I think that covers it...