Ancient Gaming Noob gains a level using the Looking For Dungeon tool and concludes that DPS classes are interchangeable. Running through an instance several times with the same tank and healer, but different DPS, could superficially make DPS look replaceable, but there are several factors overlooked. Lower-level instances are significantly easier to run than anything near a level cap, and Scarlet Monastery is no exception. There are few real challenges to be overcome for anything but entirely new players, making differences in class fairly irrelevant. It is probably possible for warlocks or hunters to tank and heal with their pets in such low-level instances, such is the lower level of challenge.
DPS characters are not the only classes that can be swapped around with no discernible difference. I have played as and run with warrior, death knight, paladin, and druid tanks, all perfectly capable in any level of instance. I have also run instances with druid, priest, paladin, and shaman healers, none of them struggling more than others. I can't enumerate the specific tank/healer combinations I've played alongside, but that's because no particular combination has been conspicuous for its competence or otherwise. This indicates to me that if DPS classes are interchangeable, then tank and healing classes are equally interchangeable. But this should come as no surprise.
The reason why the classes are interchangeable is because of all the specific effort made towards balancing the classes. Although there used to be an emphasis on classes filling certain rôles—different classes having different crowd control, AoE attacks versus single-target damage—most classes have been sufficiently homogenised so that one does not have a significant or obvious advantage over another. Many pages of internet text have been dedicated to ranting and whining about why one class is too powerful, or that another has been unfairly nerfed, so it's unsurprising that the approach to balancing the classes has been to make them more interchangeable. And it really should be no surprise that a game design aiming for balance should result in similar experiences independent of the core party composition.
Ancient Gaming Noob continues his thought by linking to a guest post at World of Matticus which suggests the controversial idea that tanks and healers should reap greater rewards than DPS classes. Because tanks and healers are necessary for any instance run, the argument goes, those players who take on these more important and stressful rôles should be given a greater incentive for their efforts. I find I must agree that the tank and healer are necessary. Although I have been in a few unconventional groups, most notably three mages and two hunters completing The Stockades at the right level, the only group that continues to be viable at any level will contain a tank and healer. Whenever I had successfully paired up with another player for some low-key instance runs, it has always been as a tank and healer duo, and we have always prevailed. I have no doubt that tanks and healers are vital for success. But it doesn't then follow that DPS are useless.
I remember receiving a vivid demonstration of the utility of DPS. I am running around Outlands killing pigs with my healer companion, and I am feeling effective and powerful. No pig can stand in my mighty warrior path! Whilst we are on our rampage, another friend turns up, playing a DPS class. My happy little sword-and-shield world of feeling powerful rapidly diminishes, about as quickly as the pigs fall under the dual blades of our newly arrived friend. Instead of each battle allowing me to practice my PvE rotation of abilities, I barely get a couple of hits on each pig before it drops. The DPS not only increases, but soars so that I am almost redundant. This increase in damage output is only amplified in instances against elite mobs.
As a quick calculation, based on abstracted numbers, if a DPS class doubles the damage output of a tank, the mobs are defeated in a third of the time. Adding that one extra player reduces an hour-long run to just twenty minutes. Filling the party with two more DPS classes increases the overall damage output significantly more still. Assuming DPS classes, along with tanks and healers, are interchangeable, a full party will complete a run easily several times quicker than if just a tank and healer attempted the same run. Quicker fights also make tanking and healing easier, less stressful. And this is without accounting for specific fights, where a boss mob will 'enrage' after a set amount of time and wipe the group if not defeated quickly enough.
I admit that healers and tanks are vital, but to suggest that DPS classes don't add an equal contribution to the group is ignoring the value of time. If your time is unimportant, it is likely possible to complete many of the challenges with only a tank and healer pair. But for most people, time is a valuable resource, and being able to make better use of time makes DPS classes just as necessary as healers and tanks. To suggest that DPS classes are anonymous or unnecessary is to ignore the relief of stress and time that comes with quicker combat. And this time is the greater reward that tanks and healers already receive when they are gracious enough to play alongside DPS classes.