The people who watch him can, and have repeatedly, actually described the skills that the possesses - and they go beyond being a big scary guy. He's dominant along the boards. He punishes players who enter the zone with their heads down.
People get so boxed in to what they think "today's game" is that they get tunnel vision regarding what skills are required. They think in terms of puck possession and transition and assume it's all slick skating and tape to tape passes. Those skills are hugely important no doubt, but they're useless without other more mundane skills in the lineup.
Board battles for example in terms of transition - the puck spends a lot of time on or near the boards, if your team doesn't win it there, your team can't use that skating and passing ability to transitioning to offense and starting a nice breakout. Think MSL in his own zone last year.
For defense, having a guy who other teams know will destroy them if they sashay into the zone with their head down is big. Again, it's puck possession and transition, but in this case defending against those things. It's great when you have smart players like Fast and Stepan who can strip the puck or jump passing lanes - but that's just one part of breaking up a good modern offense. You also need someone who will break up plays with sheer physicality, especially in today's NHL where smaller, shiftier players are making a comeback and so much offense is started from a quick transition where one or two skaters end up having to penetrate the zone early to get it all started.
His skill set is useful in todays game. It's not the absolute most useful, but we're talking about a 6D making under 1M - he doesn't need to have first pairing skills. We have three d-men right now, including Klein who so many are so reluctant to move, who all lack the "skills to play today's game" on the surface, and some of them on deeper levels. None of those guys are the board players McIlrath is. None of them hit like he can. They have NHL skills in other areas no doubt, but the fact is that the rest of the team isn't being held to the same standards as McIlrath when it comes to "today's game" by many, many posters.
As for Diaz, the guy has been taken on and let go by plenty of teams and has yet to play 60 freaking games on any of them. These preferable, applicable skills he have never earn him a permanent spot anywhere. Maybe he's being overrated just a bit.