I plan to compete in MMA (injury is derailing me for the moment but hopefully I will get to fight sometime this year) so if I get mount I'm happy. I'm just gonna pound you in the face if I get top anyway so why should subbing you be a huge concern? Dominant positions are considered dominant for a reason.
Also if you're going to be drilling subs, you need the ability to tap people out. Positional dominance is one thing, but it's not the whole of the BJJ game. People complain about people who are just content to play the position game in tournaments, so how is this all that much different?