Hi Zergalicious. 
Personally, I love the mPTs. Because they're fun and super easy to write about. The low word minimum makes it to where I don't have to count and I can just have fun with the prompt. 
To me, the purpose of the mPTs would be to encourage engagement and/or mingling. Sure, Chirper does that a bit, too, but the mPTs are a more guided way to do it. I love the icebreaker-style questions. Another one of my favourite things to do in sim leagues is ask silly questions on other people's pressers. The mPT allows me to answer silly questions, too. It's one of the things that makes the SHL so fun to me.