[GeeTwo]

First, a disclosure: I've never been "behind the glass." Our head coach did this for the first few matches our rookie year, and since then it's been all students.

If anything is done here, it should certainly be well-regulated and the channels standardized; a wild-west situation would introduce more problems (and OBTW even greater asymmetry to well-funded teams) than it would solve.
Having a coach be able to speak to his drivers over a dedicated channel would probably be good but apparently unnecessary in most cases. A more reasonable use of this capability would be to have the alliance lead (team #1 for ranking rounds, lead team for playoffs) be an "alliance captain" who would have a dedicated channel to his team captains and human players. This could either mean that the lead team gets an extra human, or that there are only two regular HP, and the lead team's HP would become the team coach in this case.
Spotters off-field sounds particularly problematic, and should be limited to a VERY small number per team (zero or one), and those should only be speaking to a coach, not a driver or HP, and certainly not the drivers or HP of other teams.