So a guild which has 20% of tanks and was doing fine gearing up in heroics and 10-man raids will have a tank surplus the moment they start tackling 25-man raids, and will have to bench half of their tanks. But a guild with only 12% of tanks which is perfect for 25-man raids will not have enough tanks to do many 5-man and 10-man expeditions.
While I would agree with Gravity that this can be a problem, I think the solution is a simple one, especially for Death Knights: The guild just needs to always invite 5 characters with tank potential to 25-man raids, and then let them organize a rotation in which half of them are doing dps with the help of dual spec.
So while the tank ratio is contributing to the general tank shortage problem, I don't think it is the main culprit. The two main tank problems are:
- Tanking is a high-stress job with significantly more responsability than dps.
- Gearing up a tank to the same level of "acceptable" gear as a dps is a lot harder, due to the relative scarcity of tanking gear.