In principal, I understand how it would reduce seek times. But with my hard drive and computer so fast today, I'm just not sure whether this time is actually noticeable. Thus, I'm not inclined whatsoever to spend money on something that might in fact not do anything except improve the situation in theory. If I buy an SSD drive and put it into the computer, I can see that my boot times decreased substantially, and my programs load instantly and virus and malware scans happen super fast. It's obvious that it worked. If I get a new video card/driver, I can see that my FPS went up in my game, or allow me to run my game at a higher resolution...thus I know that it made an impact. If on the other hand, I purchase a defragger and run it and say, "well, things should be faster now"....i'm kinda buying into it like faith.
Don't take this as a personal attack, I just questioned you directly as you mentioned a $50 app that you thought did a great job. I was just curious on what things you saw specifically to make that call. Always want to keep my eyes open, rather than just sit back and assume my stance on defraggers is actually correct.