There is little silliness quite like using a souped-up, thousand-dollar PC to play flash games, but I tend to enjoy them when MMOs run stale. As I have heard many say about no cover charge DDO, it is a low commitment, short duration, high yield gaming experience. They are intended as self-contained blocks.
Most flash games do not feel the need for padding. If you are selling a game, you add in grind, repetition, fake difficulty, and other playtime extenders, because your players expect 40 hours of play for their money. If your business model is “none” or “ad revenue plus tips,” adding in an unfun hour benefits no one. So you get not only smaller portion size but higher content density within it.
The downside is that independent trials lead to quite a few failures. If you log on WoW and run your dailies, you know what you are getting. If you play the latest featured flash game, it could be really horrible. But there again, they are self-contained, so lousy parts are not defiling a larger game/world.