I've done ST as a tank, a healer and a DPS. Tanks get in easy, healer's have a moderately good queue time and DPS get shafted. Yes, DPS. The DPS slots outnumber the healer and tank slots combined, so having more slots won't make a noticeable difference for tanks. It certainly WILL make a difference for the DPS though.

Let it go. The extra tank would be redundant in ST anyway, so you'd simply replace one beneficial job with a meaningless one.