I was JK...
The thing is, for some, having dedicated equipment like a mixer or mixing console, added of filters, samplers, effectors and the like in separate units means more flexibility in terms of use. You have more options, more control of the whole and its potential. That's what flexibility means in this case, at least for me. It does indeed, and also means more quality as once you start cramming mixers with these stuff, it's just natural that sound quality, layout clearance and other important issues may (or may not, depending on the case) suffer in various degrees.
On the other side, ain't it flexible enough to have an "all-in-one", if this piece (in the case of a mixer) is to face many different uses? Easy to carry along, easy to handle at home... more flexibility usually means more space taken and more money invested. I for my part like to own nice stuff, quality is always good investment but it does not need to be tops, 'cos as I said I use my own rig to listen and record at home, practice and do occasional gigs. An easy-to-use, good built mixer w/ some handy effects or even a sampler (like the DJM in my case) is more than enough for. And that's what I like most in mixers like the X9 and the bunch.
I could certainly do without it today as I'm playing between 3 and 4 night a week, and the venues already have that stuff, but it's nice to have some at home if you take Djing seriously. If I was to hire my equip to others, or if I was to build a soundsystem for my own bar/club, or if neither space nor money were object I'd search this flexibility myself too. The requirements would certainly change, although you will find for instance many clubs (top clubs even) sporting the DJM, the X9,
the Xone62 and the Roland 2000 to name a few that I'd put in this category.
What I mean is, ask yourself what you really need and you may decide better on the compromises that this flexibility or lack of you can take. Some things you don't wanna skip, as you said, so the rest like extra features and all is up to you and you only, ain't it?