Despite my comments on another thread in respect of cost/benefit, those comments were couched in terms of current requirements as to who can provide traffic and WX advice and the concomitant on-costs. This is a different kettle of fish.
Provided that the information transmitted is of a general nature to enhance situational awareness, then I can't see a problem with the concept.
As an ex-controller, I would not endorse the specific - like 'suggest you extend downwind,' but I cannot see a downside with something like:
'We still haven't got the traffic.'
'He's mid-downwind now.'
'Thanks, got him.'
Firies ain't dumb-bums and are perfectly capable of providing that sort of general information to enhance situational awareness. If uniformity is required regarding giving general traffic info, one could produce a very simple CD, with graphics, explaining how general traffic info should be relayed. This is not rocket-science!
As for firies attending an emergency and not being able to continue the service, as mentioned elsewhere, the understanding would be that the service is provided on the 'basis of priorities.' And that can go in the regs to provide 'legal coverage.'
I just cannot see a problem with this one as regards cost/benefit. IMHO, there are benefits and the cost would be zip!