trust me suicide oak ruins fortress matches, nothing worse than having a close fortress match get down to 1 fortress each just to have the oak player suicide to finish the last one while you are trying to kill towers jus to be able to hit theirs.
Yeah, been there, done that. We were getting creamed and I was Oak. Didn't see much to lose, so I added the Last Stand ability at first opportunity to do suicide runs. Guess what - once the other team saw that they had two players hunt me down and kill me before I even got close. Since they buffed up for the occasion, I couldn't even whap them back, so I spent my time taking giants out - better that then nothing. 
Darkside can bramble shield multiple people BEFORE they take damage where as lightside needs to wait for them to get hurt before heal is useful - Darkside wins
Oak does have Shield, but only for 1 person - and Oak get's Life Steal for everyone in range. It is not which is better, but how they play together. Rook places towers and Oak puts Death Wards next to them, the spirits that are summoned track fleeing demigods for Regulus to take out --- simple.
However, I've seen Lord Erebus and the Torch Bearer also working as a team - and when that happens - you just ain't got no hope (at least an Oak and just about anybody else). The Torch Bearer can do Circle of Flame around flags, particularly in the early game and level quite easily.