IMO - the only reason to have 5 shamans in WotLK instead of 1 tank + 4 shamans is if you plan on PvPing a lot (and even then tank + 4 shamans might be as good or better).

For leveling, dailies, and PvE, tank + 4 shamans is by far better then 5 shamans. having an extra shaman will not speed up leveling or dailies at all as 4 shamans will still do damage to kill mobs just as fast, and if you have a tank as well it will make the 5 man quests easier also (I can say there are some 5 man quests that would be tough for 5 shamans in WotLK, but not undoable).

Anyway, if your primary goal is money and PvE, theres no reason to stick with 5 shamans. If your primary goal is PvP then you probably are as good off or better off with 5 shamans, but overall I think in WotLK a tank is almost a must (and DKs have all sorts of nifty PvP utility and make great multiboxer PvE tanks as well!)