I think they should keep Seto in those spots because Seto has been better offensively in Rd 2 than Marleau was in Rd 1.
But he's even more inconsistent, so I see them changing spots next round out of necessity.
You are at least partly right.
No player, line or team can keep giving the opposition the same looks . . . especially in the playoffs. Notice how the Burger line fell apart over the course of last year. In part, they ran out of originality. Eventually defenses catch and contrary to the opinions of some, coaches are not dumb. Just at the end of the year, JT was getting blue paint duty. It's important that we watch how all of the players evolve. Part of the joy of watching hockey is observing the process and growth.
Additionally, no team is head and shoulders above the rest of the league. We are watching a winnowing process of very closely matched teams, each with a slightly different flavor. No team is going to steamroll the field like the Oilers did in the 80s and other teams did in previous incarnations of the league. What this does is put the onus on coaches for a lot more originality and tactics than they have had to produce in the past.
If others can't see why I really dislike the consistency argument from the above, then I really don't know how to help you understand. That inconsistency stems in large part from players lines and teams continually having to evolve. The more adaptable the player, the more he produces in the long run relative to his basic skills.