The stock height antenna is as tall as it is because FM radio stations are located in the high band VHF segment of the radio spectrum. (The lower the frequency is in the spectrum, the larger the wavelength, and consequently, the larger the antenna must be for reception.) So, if you run a smaller antenna, you're going to have a harder time getting weaker stations to come in because the height of the antenna no longer matches (or is anywhere close) to the
actual size of the radio wave that it's receiving.
So, if you live in an area where you're close to the transmitters for the stations that you like to listen to, and their signal is 2.5 mV/m or better, you don't really have to worry about it. However, for the distant or fringe stations with a receive strength of 0.5 mV/m or less, you may have issues getting them to come in clear when you chop down the antenna. This is one of those cases when you sacrifice functionality for style.
Here's my red cobra with a 14 inch black aftermarket antenna: