Old topic, but still relevant.
From my experience, a lot of the frustration comes from the vague definition of 'excessive' and the misconceptions around what constitutes spawn killing. So, I thought I should offer a potential concrete definition:
Excessive: spawn killing more than once once per round. I think there is probably a good argument for either one. Ultimately, a spawn kill is the reward you get for pushing through enemy lines and getting near spawn. It is also the punishment for allowing someone to break your line. However, spawn kills can be really frustrating for newer players. One spawn kill isn't so bad, but repeated spawn kills can ruin the fun of the game. There is another issue around spawn killing that doesn't get talked about very often: it changes the tactics of a given map. For example, for the B objective on Revolt, you should to have someone in construction yard to watch for spawn killers seeking around right flank. This is not immediately obvious to anyone but seasoned players. So, to make the game more fun for newer players (who far outnumber the veterans) the penalty for not knowing these routes should be lessened. Thus, a limit on once per round will make the game more enjoyable for newcomers, while retaining an incentive for more advanced players to aggressively push through enemy lines on both offense and defense.
Spawn killing: killing players before they have a chance to commit to particular lane
Let me know what you think