Oh I know the game's been more balanced now than it's ever been, more or less.
I'm just stating what the data's backing: Frost DK is scaling really, *really* bad. It went from being very competitive (I don't say top, I say competitive, there's a huge difference) in T14 and now in T15 it's bottom of the barrel with Arms Warrior. You get a lot more ilvl and your dps barely increases - it's the same that's happening with Fire Mages. Once you get to a certain crit threshold, you're doing massive, *massive* DPS. Before that, you're gimped. The same is happening with frost DKs right now - haste, their primary stat, doesn't scale well at high levels, so your DPS increase doesn't go that high (str is good for Frost but not as good as haste for 2h. For DW you get mastery, which helps is a bit better, but it still is too low).
All the data I've seen doesn't suggest capping hit and getting more exp. Seriously. The only thing you lack from not being hit capped is your RS *sometimes* missing. It's a small hit to resource regen, but it's not *that* big of a hit. It's certainly not enough so that it's worth it, imo. And from every piece of data I've read, the problem is that if you do cap hit, you run into other issues with mitigation, so while you might make your rune regen a little bit more efficient, you're losing out on mastery/parry/dodge/sta which just isn't worth it. That's the thing - if being hit capped means you get what, at best 3 more DS during a fight, it's definitely not worth it.
Let's look at it this way. 20% crit is essentially a 10% DPS increase. It's definitely nice, but it won't make everything magically better all of a sudden. If it has 100% uptime, I can expect to do what, 5k more dps on a fight? That's all it is, though, more DPS, which is a problem but it's not the root of the problem.
Glyphed DRW is going to be a little bit better in 5.4, but as it is now DRW is a shitty, *SHITTY* cooldown for threat, because it requires you to pool your runic like crazy, which is the main source of threat off a tank swap (spamming RS). When it doesn't have a runic cost, sure, it's very helpful. The only time where you truly have threat problems with the DPSers is on initial aggro - we have a fire mage who goes batshit insane during pulls and we have to be super careful because when all his procs line up he does MASSIVE damage. After that the problem isn't dps out-threating us, it's the other tank who has to sit there and do jack shit. My Warrior co-tank is hit/exp capped and I still have to be careful because it takes a little while for vengeance to drop off.