Some may disagree with me, but here is how I see this topic:
I know a lot of guilds do the "gear the tanks first" thing - Doesn't gearing someone out usually result in them burning out faster b/c they don't need anything.... I know with out good tanks you can't move forward, but last I checked our healers were not letting tanks die because of their lack in mitigation...
In fact... we hardly ever wipe because our tank isn't geared enough or our healers are OOM... it's because our dps is light. How many times have we hit enrage timers or even killed bosses after the timer goes off... The times we got vashj to 2-5% wasn't a healing or tanking issue, but light dps. (arguably b/c of deaths, not gear, but still)
I understand where you are coming from because if the tank can't cut it the rest is pointless and I think you have the guilds best interest in mind, but I don't think this is a good idea. It might do more harm than good if your tanks lose interest and you have to hunt them down to get them to raid....
I think a lot of our tanks have upwards of 500 dkp... and the lower ones higher than 200 dkp. I understand where you are coming from, but are you suggesting that we let them have T6 gear for 5 dkp? Wouldn't this lead to a surplus of dkp from the MTs allowing them to buy every sellable item that comes up?
I don't think many people can out bid our warriors anyway, and the few that can would just delay gear by a week or two tops, so it's kind of a null point.
If a tank is going from T5 to T6 and a dps is going from an item lvl 110-115 to T6, I think it is more beneficial to the raid for the larger upgrade. But when you start getting into "what helps the raid more" everyone will argue and it won't turn out pretty... so what is a guild to do? Use a dkp system. They exist for a reason.
anyway, just my .02