Ok, let's see:
1) If you're only partying with DF, that's your own problem, sorry. Party Finder exists, it's part of the game and since you enjoy the "But the game was designed to be this way, you shouldn't play it any other way!" argument, well, the game was designed for people to use Party Finder and Free Companies. I see DF as a training party or for dungeons, never for extreme primals and especially not for coil. You complain about the parties you find but the reasoning doesn't stand because if the tank is being too careful, maybe there's a reason to it, whether it's gear or lack of experience or he just doesn't want to, or like Cynfael said, the run would be more efficient if the healer dealt damage in small pulls than having to keep healing the tank on big pulls with a party without big AoE (And holy is one of the strongest AoEs in the game, in dungeons you can literally use it until the mobs are dead.
2) If you wanna talk about tandem healing, why not go a step further: 2 WHMs, both pop Regen on tank; one of them goes into Presence of Mind and deals damage until it runs out while the 1st is keeping the tank healed (Which he shouldn't have a problem with since tank has 2 regens on, but if he does, 2nd whm can just pop back on, no problem); then the 2nd whm turns off Cleric Stance, starts healing the tank and the 1st goes ahead and does the same. Do you know how much damage is dealt in those 30 seconds? I don't, but it's a lot! So there, any efficient play by 2 healers who communicate with themselves. Which is something you'll never find in DF, only in organized groups.
3) I make a point to ALWAYS save a little time for DPS in every run I make. If I'm new to the run, I'll tone it down, of course, do you know how good it feels to have the tank pull full rooms in Wanderer's Palace HM, you keeping him topped and then do this marvelous combination: Cleric Stance, Swiftcast, Holy. BOOM, 4 seconds of zero damage to the tank. Ok, you don't have to switch on Cleric Stance for the stun, but why wouldn't you? It's a 200 potency AoE! Then you can even go further and cast Holy until the tank is on low HP, by this time some of the mobs should already be dead, which means less damage incoming, which means less healing required, so you have enough time to heal, if you don't, just use good ol' Benediction. The same can be applied to Scholar for Shadow Flare, although not as much damage AoE-wise, the attack slow will help even without Cleric Stance and instead of Benediction you have Lustrate.
4) Amdapor's Keep has a DPS Check, you really think a party can get through it if the healer doesn't help dealing damage with bare minimum ilvl? haha. This is just an example, but it's probably the most unforgiving one, We can also mention Titan Ex, Leviathan Ex, Shiva Ex, hell, even Ifrit HM has a tough DPS check if people aren't geared. So if we're talking about Square being a square (lol) and making dungeons/raids with strict roles and ilvl in mind, your argument doesn't stand because healers will need to help deal damage on DPS checks.
5) And finally, "The way the game was designed". Have you seen the Conjurer questline? I think someone even posted here on how that girl said she didn't wanna heal, just deal damage, and that she was harming herself and others around her. *wink wink* *nudge nudge* much? I mean, how much more obvious can Square get with that whole questline? They WANT you to deal damage instead of standing around like a dummy (a combat dummy, not trying to insult you!)
