Despite Henry Simmons making Mac a very likeable character even in his more questionable moments, I was somewhat uncertain of his purpose in the show once the "real SHIELD" reveal was dealt with - he didn't have much tying him to Coulsons team at that point, and the show quite logically had him say his goodbyes to SHIELD at the end of "Scars". But it turns out Mac does have a purpose, and an important one at that - Mac is a god-damn badass, and a great person to be around in a crisis. I can't think of a character in Agents of SHIELD that does more to stop the Inhumans master plan than him, buying a significant amount of time for Coulson and his team while also really throwing a spanner into the works of the Inhumans plan. On top of that he gets the best one-liner that Agents of SHIELD has ever produced, and really earns his promotion to series regular for season 3.
If I do have any real complaints about "S.O.S Part 2", they are few and far between. The only thing that I can thing really bugged me is the way that the show handles the resolution of Cal's story - I can't say too much without entering spoiler territory, but what I believe was meant to be a happy ending is actually rather sinister and disturbing when you sit back and think about it. Additionally, I can't help but wonder how the larger Marvel Cinematic Universe will handle one of the many threads that are left as set-up for season 3 - the back and forth between the films and Agents of SHIELD has seemingly been better as of late, but I'm worried that what should be a huge moment in the Marvel Cinematic Universe will be relegated to simply being dealt with in Agents of SHIELD, which would be a real missed opportunity.
But overall it's hard to complain about a season finale that offers as much as "S.O.S Part 2", an episode that is able to conclude the seasons overarching plot in such a satisfying way while at the same time being full of moments that fans of the series will love and will be talking about for a long time yet, from great one liners to sudden twists to exciting fight scenes (Skye gets another pretty stunning fight scene here) to what is ultimately a surprisingly touching conclusion to the season - before a gut punch of a cliff hanger takes you completely by surprise.
I've said it before but I'll say it again - Agents of SHIELD has more than proved itself of being a worthy part of the Marvel Cinematic Universe during it's second season thanks to some solid writing, interesting characters and a sense of pacing that, although still leaves something to be desired at times, is heads and shoulders above that which came before it. If you haven't been watching Agents of SHIELD for some reason, there has never been a better time to catch up. Season 3 starts in September from what I am hearing - and I couldn't be more excited to find out what happens next.