Whether it makes sense for Pentax to join m43 depends on a number of factors such as:
a. How much Panasonic/Olympus want to charge them to get on board
b. Where they would procure sensors for it (and would Sony not cut off their APSC sensor supply if they did join)
c. How viable establishing their own standard is
d. How attractive joining some other standard (e.g., NEX or NX) is
Obviously mirrorless is the future and necessary to avoid being relegated to a shrinking niche company. We've argued that a year ago and
had a long thread where many people disagreed. A year has passed, and I think it's clear the writing is on the wall. At the very least, a lot less people would disagree now.
Pentax is late to the game so establishing their own standard won't be easy.
To be fair no matter how much I'd like Pentax to join m43 their best bet would be to join the (allegedly* open) NEX standard. With the information I have, were I on their board, I'd vote for that. Make it WR and WR lenses for a unique offering; use the excellent Sony sensors they already have access to (although it can be debated if Sony would give them best sensors if they were such direct competitors), and they could be serious players in the mirrorless game.
* This all assumes NEX (or should I say Sony E-mount) is really an open, royalty-free standard. I read that it is on another forum, but initial Google searches did not find proof of that, so it could be that the famously proprietary Sony is keeping it proprietary (or perhaps has it documented, but still charges lots for using it).