1) You void tour warranty.
2) You loose low mode.
3) Until the 14500 drops to 3.6V or so you are direct driving the LED.
4) You may burn out the circuit board. (That has happened to a few L0D CE/10440 now.)
5) If you have the RB80 version do not try it. The low Vf, small size of the rebel can't take the load. Q2 version only. (I have used my RB80 just long enough to take beamshots, so it probably won't instaflash.)
People pay extra for a Fenix because they are more reliable than the cheap stuff at DX. Put the wrong battery in and you can't rely on the light anymore.
Why not just buy a light rated for 14500 instead of trying to get one not designed for it to work.