1) They are slightly longer, larger in diameter or swell after multiple recharges and they can create excessive pressure in the laser and get stuck in the laser and that creates a very bad situation.
If there is ever any resistance as you insert your batteries into the laser, do not push them in.
2) The negative end on most rechargeable batteries is all bare metal and that can cause the battery to short circuit inside the laser.
3) Lithium = 1.7V, Alkaline = 1.5V, Rechargeable = 1.2V Double that (because of two batteries) and you just robbed your laser of a lot of power, around 20%.
That will cause your laser to under-perform and act very weak.