Like most rule changes, this petition has drawn fierce opinions both for and against it. The problem is that most of the arguments against it are not technical in nature, but against the idea that wide modes will be allowed on the digital sub- bands and thus cause interference.
Ironically, this petition will impose a bandwidth restriction on the HF bands for the first time. As the rules stand now, you could have a 6 KHz wide digital signal as long as the symbol rate didn't exceed 300 baud. The new rules will allow any baud, but the signal can only be 2.8 KHz wide.
The thing with both of these signal properties is that they are effectively self-limiting. Higher baud rates are subject to fading and multi-path distortion so they are effectively limited on HF, especially the lower bands. Bandwidth is limited by the standard SSB radio bandwidth of about 2.8 KHz, with most being closer to 2.4 KHz. Granted some SRD radios could have wider front ends, but this is far more the exception than the rule.
What opponents miss by their disdain of Pactor 4 (the most prominent mode that is driving this petition) is that increasing the baud rate can actually lead to a variety of faster modes with less bandwidth. Most multi-tone modes can use fewer tones at a faster symbol rate and move as much data than they could with more tones at a slower baud rate. This would allow a reasonably fast data stream in a 1 KHz bandwidth that would deliver a higher power density and eliminate the need for a 2 KHz wide mode running at a lower baud rate. This is effectively the difference between Pactor 3 and 4 in general.
Of course some argue that we amateurs don't need to move data that quickly on HF. Pactor 2 and Clover II are plenty fast with their reasonable 500 Hz wide signal. To this I have to point out that amateurs are trustees of a large swath of lucrative radio spectrum. If we cannot justify our continued use of these frequency bands, then the government is liable to sell it to the highest bidder in the name of the common good. Since radio amateurs seem to be falling way behind in the development of the radio arts due to a variety of factors, the ARRL has taken the tact with the government that we are a pool of trained communicators. We can provide government and non-government agencies with communications capability at little cost in times of need. Part of this scheme, and one of the most crucial, is supplying long-range email capability on HF. This requires more than the 800 bps than P2 or Clover can provide. Since a minor rule change will provide up to 5000 bps via Pactor IV and other similar modes, they are taking the steps necessary to make it happen.