Exactly. The rare "long fight" is in my opinion a good thing... because it forces the players to think differently and use their powers and abilities in ways they normally do not. They can't just play on autopilot.
Obviously you don't want to do this all the time (or even half the time for that matter)... but varying encounters in power, length, type or number of enemies, terrain location, and including skill challenges within it all the ways we have to stop "grind"... which when you get right down to it is just feeling something has gotten repetitive. And a long fight where the party is down to their at-wills and no healing can be exciting when it's never happened to them before.