It's a matter of what you can put up with and what you can afford, really. If your current bike is generally hideously uncomfortable and you have the spending money then of course you may as well look into the new bike. (Don't forget to get the shop to fit you properly at the time, of course!) Yet if you don't have that much cash to spare and the bike is causing more specific issues (e.g. pain in left knee but not right knee), then the fitting might be the more appropriate option. And if you fall somewhere in the middle of all that, well, I guess you have to weigh up the pros and cons and see where you end up.