But this is not true!
Nobody does not get to go on Soarin' because you didn't use that fastpass. All it means is that one person from the standby line will get to go in your place. It is not an issue at all. No need to feel guilty - on the contrary, instead of using it for yourself you just gave up your seat to somebody else! You made somebody happy in the process.
Ridiculous. It's not that black & white. Nothing is, and only ignorant people think anything is ever black & white. They may not be denied a ride, but they are being denied a FastPass. Say the OP gets a FP. Doesn't use it. That's one less FP. That means the person from the standby line who gets to ride faster is the person who wanted the FP who now had to wait in standby because someone wanted to keep a FP for a souvenir and there weren't any left for distribution.
Obviously, that's an extreme example, but for TSMM and at times Soarin', that can happen. Will one person make a difference? No. But it's one thing for someone to inadvertently or through no control of their own to miss the FP window. It's quite another to purposefully take a FP and not use it. If that became widespread enough, it would really cause problems for people who want FPs but can't get them.