To people who have absolutely no idea what they're talking about this is FUD. To anybody who actually thinks about it, it is making a lot of sense. OS X does not have any uninstallers, it's up to the software builder to include something like that and most don't. This will cause a lot of pollution because things like launchagents, pref files, etc. are not removed upon deleting the application. When using things like macports this can cause additional pollution of the filesystem (check the manual, there is an autoclean option). Such leftovers normally don't cause any problems, they just sit there taking up space. Sometimes those leftovers can actually harm the system and cause some problems (which you can check in Console).
The above is the main reason why tools like AppCleaner, Onyx and OS X's own maintenance scripts (daily, weekly and monthly) exist. It's also the reason why some software builders put uninstallers next to the installer in the dmg. OS X's own maintenance scripts run at certain times (past midnight) but can be triggered manually by using tools like Onyx or simply by running them from the command line (Terminal). AppCleaner and the like will delete pref files and such whenever you delete an app by putting it in the trash. However this does not always remove everything, sometimes launchagents are not removed.
It's not something you really need to worry about. Throwing away pref files when deleting the app is also very useful when you delete it because you want to fix some problem with the app. In case of the topicstart this has absolutely nothing to do with OS rot or any other slowdowns with the Mac mini. The mid 2010 MBP simply is an awful lot faster. It's not about how many MHz faster but about how fast it handles threads and things like that. MHz do not matter any more, it's about what it can do in every MHz. I have a Mac mini early 2009 with the 2 GHz C2D, 4 GB of RAM and a ssd. It's a snappy little machine. My mid 2010 MBP is the cheapest 15" version but I upped the RAM to 8 GB, it has the same ssd as the mini has. The machine is unbelievably faster than the Mac mini, simply because the Core i5 is incredibly fast compared to the C2D. Especially the improved virtualisation support in the Core i5 makes a lot of difference in my case (as well as the 8 GB of RAM). The TS does not have this Core i5 cpu but his C2D is faster than his Mac mini's as is his IGP (the 320M). In other words: the mid '10 MBP simply is a faster machine than the Mac mini '09.