I personally was fine with it mp4 subs, but in some games you want special placement for them, otherwise they may cover all the ingame text. Standalone players can override subtitle placement, but for streams there's little to no point in such subs if they cover important gameplay elements. In mp4 you can't change their placement or embed any other subtitle format. I think it was Spike's idea to use mkv for 512kb.
And as I said, if 512kb encode is anyway done as mp4, there's no point in having it as mkv as well. For proper subtitles one needs mkv, and I see no benefit in having 2 mkvs. So I'd only leave primary mkv, and if subtitles don't look right in a given game using mp4+srt, then only mkv should have the subtitles.
See
[2719] Genesis Langrisser II "all levels" by ars4326 in 1:56:46.59 as a great example.