This is huge and is a great write up from James Wilson from bikejames.com and it totally reiterates the point that I have been trying to make. The rider makes the bike not the bike makes the rider and I feel that unless you're bike breaks or is at least 5 years old or you have money to burn you really don't need to buy a new bike or upgrade your bike unless you reach the point when the bike is holding you back from progressing. This could also happen if you change from XC racing to DH but most trail bikes that we have will suffice for both unless you are racing at a high level but if you are simply doing your local races you will be fine. Can a new bike or part make you a better rider, nope, but can it make you feel like a better rider.. Possibly but it might also then be covering up a weakness. James talks more about this.
I also think sometimes it's nice just to get something new as it's a bit of confident booster. More like a placebo effect then actually covering up anything, but I tread lightly on this subject because new isn't always a bad thing, just something new! right? Hell ya!
Check these two articles out, they are must reads in my opinions