the taller the plant gets the less suport it has to keep itself up. it will just fall over. then when you flower it won't have the suport to hold up the buds. with the buds laying on your meduim they are more likly to get mold or rot. nobody wants that except the man.
On top of what the others have rightly said stretching creates another problem,once you start to flower you want the nodes to be very close,the closer the nodes the bigger & tighter the buds will be.
If you find they are already stretched you can also take em out of the hole and poke the extra stalk down into the medium leaving the leaves above grow level. Did my two like that and didnt hurt em a bit. That stalk just turns into mo roots. I forget who suggested that now. Tek maybe? Thanks to whoever it was. Anyway that is a good way to cure the problemo. Mine was real stretchy till I done that. Aint a bit stretchy now.