For ordinary consumers, the foremost criteria in buying a bike may lie primarily on the aesthetics like color and graphics or some other considerations and with the “bike-fit” as something lesser down the list of criteria, if it matters at all. With the help of the shop sales person, the sizing decision maybe decided via a quick and simple assessment of the height of the rider vs. the general sizing guidelines like a 5’2″ would warrant a size small bike, a 5’8″ a medium and so on. And yes it will work and in fact sizes can be adjusted through the “adjustable” components like seat post and/or easily and cheaply replaceable parts like the stem for the stem length. But for the more advance and technical user, bike fit is the utmost criteria, without compromise. And these makes more sense when we are talking about huge amounts of money in investing in high end products. Being precise about it, not all people with the height of 5’2″ have the same length of legs or torso and so on. Some even have an uneven length of legs. And there are even other factors that are not measurable but can be a preference that only riding experience would tell like a spinner (comfortable with light gears but requires more crank spins) and a masher (rely on power so he uses a heavy gear with fewer spinning) may have different crank arm length requirements.
When you have long riding hours under your belt, your body can already appreciate a good fitting bike from a bike that is just trying to fit in. These can be manifested in terms of comfort, handling and efficiency. In the pro leagues like the Grand Tour, riders are known to be so meticulous that they can smell the difference in adjustments to the millimeter levels. If you buy a wrong fitting bike, yes you can change components later on to fine tune the fit but if the frame is the culprit itself, it may be a costly replacement. Trying to adjust on some particular components only, even if it reaches the right intended measurement may have other implications (i.e. a too long stem to fit your measurements will have impact in terms of bike handling).
So a good buyer should be aware of his own size or fit requirments (in details) when buying a bike. There are general accepted methods in self measuring your fit and you can download such procedures from the internet. But for the more advance and meticulous, it’s better to do a bike fitting session done via known bike fitting systems. Here in the Philippines, the use of these bike-fitting tools may cost you but that cost is worth it, as these measurements will be your life long guide (unless you jump from a tall building decreasing the length of your legs). With the 1K-2K cost, it will give you these valuable numbers. It may not be 100% accurate for you but it will still be the best and scientific starting point.
I had my bike-fit done by Toots Chua (3T, LifeCycle) more than 8 years ago. I heard Bike Town Cyclery of the former COMELEC Commissioner Goyo Larrazabal also has its bike fitting services using the same fitting system as that of 3T. I remember when I had mine; the equipment was in transit between shops so I ended up being measured from Mr. Chua’s residence. I had 3 documents then, a mountain bike cross country “competetive” fit, a road bike fit and a cross country fit. Nowadays, there are more complicated tools like 3D and computer aided tools beside these baseline numbers, all for the sake of getting as close to the right fit for you.
So if you are on the serious side- pop out that 2K pesos and I’ll tell you, it will give you more comfort, albeit sometimes more on the psychological, but either way, it does matters. For me, the most helpful and most used number I got from that fitting is finding out that my saddle height is 72 cm from the bottom bracket thus everytime I adjust my bike or even when I borrow another person’s bike, all I have to do is measure 72 cm and I’m set to go.
(The system that was used was from www.bikefitting.com. Visit the site for more helpful bike fitting tips. To further fine tune your measurement, you can buy aide products from thsi site- http://www.bikefit.com/index.php).