Well speaking for quicksilver, the advantages are that you can not only launch any application or utility you want, but that you can also open any document or execute (nearly) any command you want too.
For example: I can bring up a document with a few taps on the keyboard, and move it to my external drive, email it to someone, upload it too my FTP server, or open it in any program I wish (not just the default setting).
All that is REALLY easy to do with QS.
I can also map certain commands to certain keyboard keys:
I have F1 as track back, F2 as play/pause and F3 as track forward (for iTunes).
These hotkeys work system-wide too - regardless of which application is currently active.
I can also navigate iTunes like my iPod through the QS dialogue: again all with a few key combos.
/end QS loving