Many people with Steamos still just play on keyboard and mouse. I know one answer would be to just "start a game in desktop mode" but gaming mode is still different and, I think, should still be fully usable with kbm on its own.
For me this is especially an issue for non-steam games in gaming mode. in some cases they become basically unplayable. In other games, such as rocket league, the onscreen keyboard becomes more of an annoying popup than anything useful. Having an option there to simply disable it would very nice.