The problem is I haven't seen anything convincing from Roache at first grade yet to really justify this. Plus Luke should be an 80 minute player. If anyone is worth having in bench cover is Tevaga, who has really looked the goods and can also slot into the back row.
I see where you're coming from - but I'm not sure (looking at him as a hooker) Tevaga has done much more in first grade - and of the two, Roache looks the better DH option when they play NSW cup. As for the back row cover, I don't think it's quite as important as covering the spine/backline, since we have the minutes in the other guys there to cover for an injury or two in the forwards... 3 starters who can play 80 if needed, plus Gavet/Lillyman - and Matulino when he gets back - have all shown they can play significant minutes to good effect at this level, Lisone is a guy I suspect is better as a longer minutes player than an impact guy, and Sao has been getting barely any minutes, plenty of room for more there.
Certainly trying to use that last guy as an impact option (which I'm guessing is what Kearney is going for), Roache is much more suited to that than Tevaga, who is a workhorse but not particularly dynamic... Roache likes to run, and he's good at it. Also I feel Luke is maybe not at his best when playing the full 80 - he certainly has that ability if required, but his workrate in defence isn't exactly great for his position, he averages more like 70 minutes/game over a season anyway and has I think looked better for the Kiwis when in rotation with TL.