Of course, 6th is also the level where the mass combat buffs start up - Need everyone to have better AC and Reflex? Cat's Grace, Mass. Will saves? Owl's Wisdom, Mass. HP and Fort? Bear's Endurance, Mass. It's also got one of the strongest Damage-Dealing spells around - Disintigrate (2d6/level, but single target). Mass Suggestion is really nifty, especially with the save required for a level-6 spell. Blade Barrier makes for a nice defensive position for a time, Heal gets people fixed up fast, and Mass Cure Moderate Wounds is better the more people you can apply it to. Planar Binding and Planar Ally can get some strong backup.
Sure, Hero's Feast is a rather strong protective agent - at least, against what it protects against - but then, there are also some strong offensive agents out there at the same level it's available. Moreover, it's got a long casting time (10 minutes to cast, 60 more to gain it's benefits), can be interrupted under most circumstances (picture a Sorcerous enemy sending Demons and Devils bound with Planar Binding to the PC's while they are eating - rough them up, possibly catch them off gaurd, AND make them waste a casting of one of their higher level spells; Mage's Magnificient Mansion (7th level spell) but little else can), and is mostly only stopping some save-or-(effectively)die effects that aren't likely to come into play every session. Sure, the +1 Morale bonuses are nice, as are the temporary HP, but not really anything that can't be dealt with by an extra encounter or two.