I still dual boot XP and windows 7, and XP is still my main OS.
Many elements in the OS have pretty much instant responses while windows 7 (and also windows 8), there is a noticeable delay. when you regularly use both, even though those delays are not long, it is noticeable and annoying because of that, (even with a SSD)
Old is not a good enough reason as to why someone should not use something, if thats the only reason you can think of then you have failed to understand the purpose of the OS.
If you have a modern PC, try running windows XP on it and you will see how an older OS on new hardware runs as compared to a new OS on new hardware.
here is a video of some basic things done on windows XP (running in a virtual machine using a fairly modern PC)
https://www.youtube.com/watch?v=gLsNxKjp9L8
pretty much all of the security issues that get discovered now either effect windows xp, vista, and 7, and when it is not effecting all 3, it is busy only effecting vista and 7
So security and general performance are not reasons since windows XP has proven it's self to be more secure when it comes to protection against exploits that do not rely on social engineering. (XP simply has less exploitable code running.
Other than that, think about things that you are currently doing on windows 7 that you cant do on windows XP
The only benefit that you really get is proper 64 bit support with a very minimal 32 bit performance drop (32 bit apps do benchmark lower in windows 7 in many cases, but it is generally only by such a small margin that it is impossible to notice it outside of a benchmark)
Simply put, many parts of windows XP require fewer IO's to function and load, and with the fact that it is faster to load less than it is to load more, having a smaller OS on the same hardware will = more speed, at least when it comes to IO intensive tasks relating to elements of the OS.
(For general computing and lite gaming (dx9 games, eg TF2, I stick with windows XP) for professional apps like photoshop, and video editors and 3d modeling applications, I stick with windows 7 because it can use all 12GB of memory on my current system, (XP 64 bit is just garbage, it handled 64 bit apps well but failed horribly when it came to the way it emulated a 32 bit environment (massive loss of performance while windows 7 has a loss so small that you cant notice it outside of a benchmark)
Remember, windows XP was designed during a time when hard drives were extremely slow, and now that modern drives are much faster, things just become pretty much instant.