I think there is a significant difference between a weak two in diamonds and a 2♣ weak two in diamonds (that may also have some strong hands on occasion). The former exerts much more pressure. The latter frees up a bid for a multi, but it achieves different goals. At some point I was a big enthusiast of cramming more and more hand types into weak/strong bids, but at the table they just didn't seem to win as much. I'm now a big fan of weak bids where responder may frequently pass. The easiest way to do this is by including the suit bid into the description of the bid (although even a 4-card suit might be enough, on occasion) and I believe the weak-only multi players are trying to recover the same threat through different means. Multi is a fun convention and certainly has multiple upsides, but it also comes with noticeable cost. If you want to open more often, or preempt more often, I think multi is not at all a great way to do it. There are lots of interesting and aggressive preemptive schemes with higher frequency and, in my opinion, lower cost than multi + some weak uses for 2M.