well, when someone is mortaring in spwan just block him or kick him out or whatever. and since mortar out of spwan is not allowed i see no reason why it should be allowed to kill the mortar guy in the spwan? prevent rulebreaking by rulebreaking? you are able to punsih him if he does it, so do it!
and when you allow light sk you can be sure that this will end up in nade spam on spwan, poison gas and so on...
like marx said - when i want to get killed in spwan or have team damage on then i play et pro...