I've been watching forums for a while now, and it seems that the trend is for people to demand that every GCD has to be filled. But that leads to a problem with certain archetypes.
Consider 10 GCDs. Let's say that each class is designed to deal 1000 damage after 10 GCDs. If every GCD is filled, then each button press contributes an average of 100 damage.
But say you want a class that does big hits. With an ability that does 400 damage. That means the remaining 9 buttons only do 66 damage. So you're spending button presses on many weak abilities just so you can have that one big hitter.
There are ways around this of course. For example, the small ability could boost the large ability, so a portion of that 400 damage is really attributable to the small ability.
But overall, it seems easier to design a class that doesn't use every GCD, or has abilities that cost multiple GCDs (casters, usually). If you only used 5 abilities in that 10 GCD window, your heavy hitter could do 400 damage, and the other four still do 150 damage, which is more than a class that fills each GCD.
Then the problem with empty GCDs is that filling them with anything becomes a viable means of increasing damage. The usual route is to throw in AoE abilities, like Retribution paladins did with Consecration.
SWTOR provides all classes with a very weak filler ability that does not cost resources (or even grants resources), giving the player a button to press when they have nothing better to do. I'm still not sure it's better than just having the player wait for the extra GCD.
Ultimately though, is a class with empty GCDs a viable playstyle anymore?