Personally, IMO the only reason there aren't more people in the MW is the lack of uber planes...guess you can't have "good fights" without the uber planes can ya.
Amazing how 10 people in an arena can dictate the "fun factor" and yet there are thousands of whiny posts regarding the LW arenas too. Want change? Get more people in the MW arena...oh wait, I forgot, it's easier to blame someone else for one's own shortcomings.
Not just addressing Cap and Masherbrum.
actually, i have tons of good fun fights with "not uber" planes. i've been flying the 38J and 38G almost exclusively. they(in my book anyway) are FAR from uber.
always get fun fights against the likes of ump40(kicks my cartoon arse 99% of the time), llgaf(50/50 win/lose), potsnpans(insists on using that dam 205 to break my airplane

), and lately, Aarrg(fought him one time in a 38, the rest in his zeek, which he handles very nicely), and a few others.
all nice guys to chat with on open channel.
i do have to confess though, i've been flying a bit higher than i used to though....but that's dictated by where i normally find the fights.