As it's written a medic is probably more useful to a larger party than a small one. Although I can also see a use the wider use, healing comrades and NPC's.
I have a couple of questions with how it works now. Firstly, is stabilising a critically wounded character included in the 24hour limit? It shouldn't be and that's enough reason to have a medic around in a dangerous game like OW.
Secondly does a successful diagnosticatior use actually help further First Aid tests? I can see that it would useful in other situations than directly healing but I'd normally give people bonuses to tests if they had correct supporting information but it's not mentioned in the description of the Diagnoticator or in the difficulty of the Medicae tests. If it doesn't mention it anywhere I'd be inclinded to give a+10 for a successful test and an additional +5 per DoS (up to +30 max).
Personally I can a lot people using house rules until it's gone into more detail, I think there's a good chance of that being included in Hammer of the Emperor if they flesh out the specialisation. The simple answer would be that a character that had first aid used on them cannot be treated again untill they are wounded again . More realisticaly, once treated they cannot cannot be treated again untill they are wounded again and this cannot remove more wounds than they have taken since being treated.