for me there is good power leveling and bad power leveling.
A good power leveler will reduce down time by curing tanks inbetween fights and giving them the best buffs availible, but only after the whm has casted their lower lvl buffs to raise their enhancing. they don't cure or only use cure spells apropriate for the level and never over cure. They raise party members in the event someone dies.
A bad power leveler will over cure to the point of where they are tanking. This is bad. 2-3 lvls of a power leveler tanking can seriously under level a Palidins shield and parry skill. It can set a ninja back as well if they are not getting their evasion points. It also doesn't help new tanks learn to do their job. Mages won't heal and therefore their healing skills will fall behind. Everyone looses in this situation.
Power level if you want just don't do it badly. It hurts everyone.