In his post, Gravity looks at the number of tanks needed for groups that vary in size from 5 to 10 to 25. He notes that the proportion of tanks needed for 25s is lower and remarks on what this means for tanks, particularly in terms of getting raid invites.

My guild handles the situation a little differently. Rather than bringing fewer tanks, we take the same proportion of tanks to 25 raids. Also, due to a somewhat unusual loot system we will frequently have an additional two raiders rolling on tank gear (ie, 6 people rolling on tank gear). The loot tables for 25s, however, are built to support roughly 3 tanks, so the chance that an individual tank will get a given piece of loot is substantially lower than expected. So, in other words, the situation with respect to raid invites is ok but is abysmal where loot is concerned.

Has anybody run into a loot system like this? It seems backwards and unfair, but I suppose I am biased since I am one of those getting screwed over. I’m at my wits end regarding how to get some change. I tried posting on the guild forums in a very careful, understated, unemotional and non-accusatory fashion but that turned into a big furry mess with no resolution. Help!
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I also agree, like Falc said, that it’s the demand for tanks in raids, combined with the gearing problem Spinks mentioned, which creates the number of tanks who are available to pug.  If there was more demand for raid spots for tanks, there would be more tanks around altogether.

Veneretio’s posted on this topic too; his position is we should have more multi-tank encounters and gave some examples. Some of his commenters’ views are also notable in their own right.
Next, Tobold posted that he believes the main culprit is the difficulty of gearing up a tank to the necessary level.

I wrote about this (or something similar) awhile back – the great tanking problem.

But also, because raids need varying amounts of tanks (usually between 1-3), tanks really need a raidworthy offspec. So as well as gearing up as a tank and learning to tank, you also need a dps set and to figure out how to raid as dps. I guess there are some groups that always force the DK or druid to offspec but in ours we take it in turn. So it’s so much extra work compared to what a healer or dps needs to do.

It’s tricky because it’s based solely on the content. Given only a set of classes arbitrarily assigned abilities, there’s no way to determine if they’re balanced for PvE. You need to know what you’re fighting in order to establish some understanding of how effective the available strategies are. Without enemies, there can be no fight, and with no fight there cannot be viable strategies to beat that fight, and without viable strategies, there can be no conception of balance. So the PvE balance of a game entirely depends on content–not on available content, but viable content. If there are 90 raids where a DK is a better tank and 10 raids where a Blocking Warrior is better, the game is unbalanced if no one actually plays those 90 raids where the DK is viable.

PvE balance is soft because the metric of success is binary. Efficiency doesn’t matter much beyond if the whole raid gets through the content with no wipes (and in a reasonable amount of time). So if a sub-optimal tank can tank well enough that there are no deaths because of his play, that sub-optimal tank is actually balanced against the optimal tank in a way.

So how can you determine if different kinds of tanks are balanced? If it’s viable to use either in some role (I’m accounting for multiple required tanks here)  in a roughly even number of instances, then it’s roughly balanced. This doesn’t only change with class ability changes, though, it also changes as new content comes out and old content because obsolete.
(I may turn this comment into a blog post in the near future–you have been forewarned!)

So i completely agree with this article, the fact that 25 mans have such a small need for tanks, really limits the number of tanks out there, even IF the gear itself is there, you won’t really feel well compelled to do something you won’t ever be able todo in a real raiding environment.