It's in a game-designer's interest to make their games as habituating as possible; that's why you've got things like raids in World of Warcraft that only drop loot once a week: they want you to carve out at least one night a week for them. Other games are more aggressive and try to make your game-play fall at the same time every day (e.g.: Farmville) The way they get you habituated is with a reward for being a habitual player. Rewarding the habitual player can be seen as punishing the 'casual' player depending on whether you're a glass-half-full kinda guy or a glass-half-empty kinda guy. Done badly this can drive players away in droves; take for example Warlords of Draenor's tamagotchi-like garrisons, which sucked up so much player time that they killed the social content.
In principle I have no problem with that; it's implicit in game-play (as is "grinding") and the only effective way to push back on it is to vote with your feet until game-designers either stop doing it or come up with another way of stimulating obsessive behavior. So, mentally, I re-formulate your question as whether or not standing in PP is the best incentive structure for dedicated players, or whether there should be another. Personally, I'd prefer if there was a rivetting story-arc that sucked people in and didn't let them go, but that's what I value in games.
How would you feel if players' standing was affected by other players' standing? Let's say you can be in the "top 10%" of one power's supporters, or the "top 100%" that way your standing wouldn't "decay" but other players might pass you and leave you in the dust over time while you were doing something else? I suspect people would scream about that, too. Gamers don't appear to like zero-sum competitive arrangements; the "me me me!" crowd want everyone to win, only they win a little more - which inevitably results in inflation and periodic devaluation-of-everything like in World of Warcraft. Unfortunately the best way to prevent inflation is by establishing a zero-sum system.
From an in-game standpoint it's perfectly reasonable. I'd go all Darth Vader on my minions if they suddenly took a week off and didn't show up when I was in a crucial series of battles... I see the whole powerplay thing as a superstructure reward for players that want to be more important in the game. It's not taking anything away from the casual players; they're just not going after that reward.
Consider the epic cape quest in Mists of Pandaria: if you're on it, you won't lose ground from where you are, but when a new expansion drops in 12 months, all your progress becomes irrelevant because now all the cool kids are going after Khadgar's ring quest instead. That's what I mean about deflation at the top. Blizzard maintains value by periodically making all your hard-won accomplishments valueless.
Personally, I'd rather see a model where casual players actually were punished for not participating as much, than a model where everyone gets devalued every 12 months. I'm not convinced FD has thought this through, though. They seem to be heading toward the worst of both worlds: the cost of ships and outfitting will spiral up to absorb the higher payouts from missions, and value will automatically decay. That's a "deflation plus stark fist of removal" strategy and I'm not sure it's well thought-out.
