I'm not arguing that a 4th liner will produce like a top 6er. Of course playing more minutes and with better teammates helps scoring. I'm saying when constructing a lineup, your philosophy should NOT change in terms of what you are looking for in players in the "top 6" and "bottom 6". A 4th liner does not have to be a "grinder" or defensive specialist. He can be "skilled" and lean towards the offensive side.
It's not playing more minutes that make top six guys productive, it's being more productive that results in them playing more minutes.
It would be nice to have 12 guys who can score, and we might be there in a couple years, but even then someone has to do the dirty work, you don't want offense first players on the bottom six who can't contribute in other ways, they're better off playing in the AHL and becoming trade bait if they can't round out their game.
This doesn't mean you want slugs, but if your choices are Lehtera/Weise/Read/Leier, dress the one(s) who can PK because none can score.
Odds are you might get 30 ES goals from your 3rd line and 15-20 ES goals from your 4th line, in that case, a guy who might score 1-2 more goals in that role (limited minutes with limited linemates) isn't worth dressing over the better defensive player who is good on the PK.
The advantage of guys like Farabee, O'Brien, Lindblom, Laughton, ect., will be that they can contribute more than just goal scoring, they'll give you adequate offense but also contribute on defense and PK. This will be the litmus test for players like NAK, Allison, etc. - because there aren't going to be openings on the top two lines for a couple years.
Simple math, 10 ES minutes a night, 80 games, 800 minutes, 13.3 - 60 minutes.
So a guy who scores 0.30 pp/60 more than another player will generate 4 more points over the season (so 1-2 goals).
The difference between a 1.20 v 1.50 pp/60 player is pretty substantial, but in limited usage has limited impact.