I'd prefer 3/4 lines for a number of reasons.
You actually have to earn the minutes.
You will see progression being made throughout your career as you move up the lineup (rather then just 6 seasons on the top line with similar point totals)
More active locker rooms.
More line-matching (can find out who has chemistry with who, coaches will have more options to mix things up, specify against certain teams)
More trades (Some teams will look to acquire depth, while others load up on star power)
More of a team-atmosphere.
I feel like the 2-line system is just in place for people that are here for themselves. They care more about seeing there player do well, then having to earn there way to the top, and being apart of a team.
I haven't read a lot of the discussion on this topic, but that's my take. This is probably for nothing though, as we might not get the members to support it, like others mentioned before me.