There is no rule that says you need 2 sets of gear to switch between the two roles. In fact, I carry 2-3 20 slot bags full of equipment with me that I switch out a few pieces here or there tailoring the set to the current role. You can already choose to gear a "hybrid" feral that uses high armor DPS equipment. Our tier sets fall into this category, as well as pieces like [Belt of Natural Power] and [Treads of the Den Mother]. The fact that many maximum DPS pieces (ie. rogue leather) available do not have high armor is done on purpose. We need to choose to maximize damage, tanking, or hybridize between the two. You can already do what you're asking by wearing a hybrid set.

As an off-tank, many times I don't bother reaching the anti-crit cap. An example would be Hex Lord Malacrass. We often kill 3 or even all 4 of the adds before engaging the boss. I off-tank this in nearly full DPS gear, but I choose high armor dps pieces including some with extra int to allow more shifting and emergency healing.

You are asking to be the equivalent of a DPS warrior putting a shield on to tank an add. We have done this often from Mag's Lair through Hyjal, but we generally need to kill their add first. Personally, I want the ability to compete with the prot warrior, not the dps warrior. Otherwise, they will just take the warrior instead of us (like pre-BC). Being able to do this with a simple gear switch is why warriors hate us. If we could do it without a gear switch, why bring warriors?

In Item Rack, I have many sets pre-made including max armor, max stam, max dodge, default MT, default OT, max DPS, stam DPS, armor DPS, and more. The ability to change gear and change our focus is a GOOD thing.

Consider writing a macro for travel / flight / aquatic form, e.g. Useful macros for druids - WoWWiki - Your guide to the World of Warcraft

I can see someone make the argument that when they run from mob to mob, they shift bear, feral charge, then shift back to cat, essentially making the running time only the distance between you and within charge range. I won't take that any further, though, because you get into a "what's your energy when you've just finished killing something" argument that doesn't have a "right" answer.

Also, I believe that the Feral Swiftness talent, which is pretty standard for most specs, doesn't stack with run speed to boots, so it's a pure loss in Hyjal in catform.

If I used the same boots for tanking and DPSing I would want Cat's Swiftness on them even more. Take Hyjal for instance- I spend most of my time tanking on trash, when Minor Speed is actually quite nice to move around in addition to using Charge. On Archimonde I find myself in caster form decursing a lot as I move back from an Air Burst or a bad Doomfire, when Minor Speed is again helping me return to the fight more quickly and thus raising my DPS. Using Feral Charge to get back to Archimonde after you've been driven away is usually a great way to get Doomfired.

Allev: It's not necessarily a question of how much energy you have when the mob dies as how much energy you would have as you get back to the mob/reach the next mob, which is almost always 100 if you didn't switch forms along the way. This means you lose ~40-60 energy by Feral Charging. I don't know how long a cat with/without speed on boots takes outdoors/indoors to close the same gap that Feral Charge closes, but it's worth investigating.

That said, valid points.