I can only see those as working in the sense of leveling up and getting upgrades. However, I don't think they would be that great for dialogue and storylines. Because a guardsman doesn't get options, he just does as he's told. A cultist is pretty much the same way and a marine does the same as a neophyte. And Orks just fight and fight, there's no dialogue regardless.
No dialogue? I don't see how they can have 40k novels if there is no dialogue. Would make for a rather boring read.
Even the characters from the dow games have dialogue, especially in the first game.
A cultist or guardsman doesn't have options true, but if either shine then they can move on too better things. That's where the dialogue and drama would unfold. Getting to the maximum level and the actual getting there part.
Do you survive the rampaging hordes of greenskins with a wicked bodycount tally, save a commisars life and drive him away on a looted Leman Russ being rewarded a promotion or do you join a Cult of Nurgle, get the flu, infect a small populace and get elevated to Daemon Prince?
I can see plenty of action and talk.
Wasn't he talking about just the orks having no dialouge? All they say is 'Waaaggghhhh!' and 'Kill da humies!' The normal Orkish banter. I can see why that wouldn't work. Pretty much Eldar and maybe the Tau would be the only real choices for a RPG. Only problem with the Tau is the Air and Fire castes are the only ones that fight. The others would be too boring to play as.
True, the Water and Earth castes would almost be an entirely different game. Earth castes would just repair vehicles and structures and work on making installations. Whereas the Water caste would only deal with negotiating with other races and between the Empire itself. The only way those two could really tie in is if you had them be background characters you could interact with. Say for example one mission is defending a Water Caste envoy or the Earth Caste while they repair a dropship so you can evacuate.