Siege is for people that are after "epic" moments; epic clashes on top of ladders/flag, epic deaths (get a mid-air kill moments before gravity death, for instance), epic kills (nudging people off ledges mostly). No one gives a shit if they die, so people take crazy risks going for that epic moment. And you don't have to spin your camera around nonstop; you know where the enemy is, and they aren't riding a Charger up your ass.
90% of battle rounds are decided by the autobalancer, and you just have to run around with the clump as the round approaches its inevitable outcome. The only epic moments occur in the rare rounds where the last few survivors are well-matched. I do well on battle, but it just feels dull compared to siege at its best.
Mate I have about five thousand hours of crpg siege, but it is still the lesser of the 2 modes, and you still find people on siege who you could swear are convinced if they die it will kill them IRL, running for miles, and the outcome is picked as much by the autobalancer as battle.
In battle, clutching a round is simply amazing, and yeah unfortunatly for the majority of the community grind is the main aim, so they constantly stick in ganksquad, but smashing the opponents main clump then hunting down the stragglers, or being the last alive and selling your life hard for as many of the enemy as you can is awesome, and each opponent you leave in bloody tatters is not gonna respawn and come back in a few seconds.