You know, I have SSDs in all three of my systems- and all three have mechanical storage as well, one of which is attached by USB3. They all have Intel quads (2 Sandy and 1 Ivy), and two have 16GB of RAM, the other 6GB (more than enough for games). And yeah, I've played games that were loaded on that USB3 mechanical drive
So here's the thing- many/most games will see some benefit to load times, but load times are relative, both in the speed of load times, and in how often the game stops gameplay to load content. If you only load once, a 5-10 second difference doesn't mean much if you're already waiting 15-20 seconds. If you have to do regular loads, a la Skyrim, but they're only 2-5 seconds, what does dropping that to 1-3 seconds matter? You're still interrupting gameplay.
I keep throwing BF3 out there, because it takes up 25GB of my SSD and it sees a massive boost in level load times, something you might be doing quite often depending on game type and server settings. Slow load times are also frustrating as you're usually trying to join your friends.
Another example is Skyrim. Even with a stack of mods, Skyrim loads acceptably fast for me from a 2TB Green drive. Definitely faster on an SSD, but not fast enough to warrant the space, and many single-player and lighter multi-player games behave like this.
It's a decision you have to make on a game-by-game basis; I couldn't possibly fit all of my games, along with the OS and primary applications, on a 512GB drive, nor do I wish to shell out for one yet. It's also hard to recommend that someone spend another $200 or $300 just to get one large SSD when most of that expensive space won't actually be used and could be just as well be applied to a much larger $100 HDD. Take that $100-$200 savings and put it in your pocket, or somewhere it will count more, like a better GPU, monitor, keyboard, or mouse.