Well, they could still "bomb you". Just not in 1st but 2nd or 3rd turn. 2v2 or 3v3, 5v5 - those number should sound familiar, right? Whats the difference?
Yet, I still agree that loosing 100-300 ranking points in first three turns is harsh. Especialy when you REALLY get "bombed" by 3 so called "guests" at once. My first game ended this exact way which made me R:1320 after my first match with ranking

And while we're in the topic... Guests are such a R leechs! Today I've seen loads of games with up to 6 out of 8 players being guests. Even more - those were "no boosts/no ups" games so guests had the same chances - YET they reward us with no points. Why NOT to hate them?

[EDIT]
25% and 15% def boost in 1st and 2nd turn respectively
or
+3, +2 and +1 to rolled valued in 1st, 2nd and 3rd turn respectively
Buuut this would still encourage players to "block" you. Like you have just one way passage out and they place exactly there to just stop you for two turns. Yet, this would just requite smarter starting i guess.