Plouffe is right -- both for the practical reasons outlined in the quote above and for more symbolic, but no less important, reasons too.
The reality of being Hillary Clinton -- the best known politician not named Barack Obama in the country and a massive frontrunner for the Democratic presidential nomination -- is that the campaign both for and against her is already well under way. There are outside organizations designed to gather opposition research against her (America Rising) and to protect her from that criticism (Correct The Record). There is a super PAC -- Priorities USA -- that is expected to carry the load of the pro-Clinton, anti-Clinton-opponent advertising and another one (Ready for Hillary) building a grassroots network for her candidacy.
Hillary Clinton, in the eyes of the political world, is already running for president. Waiting to acknowledge what everyone already knows and has spent the last year-plus preparing for makes little sense -- particularly because running for the nation's highest office requires massive amounts of time for fundraising, organization building, message development and so on and so forth. Why play coy when there's no mystery? Get in and start running. Everybody already assumes you are.
The less obvious but maybe more important reason for Clinton to announce sooner rather than later is that it would be somewhat unorthodox and unexpected. The conventional wisdom surrounding Clinton's timing on announcing is that she wants to wait as long as possible for people to see her in a political light. Her approval ratings have been sky high for years now because she hasn't been in office (or running for office) so why not preserve that "I'm not a politician (yet)" vibe for as a long as possible?
Clinton spent the entirety of her 2008 race for president guided by just this sort of conventional wisdom. It led to her being perceived -- and being -- an overly cautious politician who always wanted to see which way the wind was blowing before making up her mind about anything. (Remember that the start of Clinton's slide in the 2008 primary was when she tried to have it both ways on a question about undocumented workers getting driver's licenses at the end of a debate in the summer of 2007.)
Clinton needs to run a different kind of campaign this time. And, she has the luxury to do some unconventional things because, unlike in 2008, there is no real threat to her among the other Democrats thinking semi-seriously about running for president. By "taking a risk" and announcing early -- although, of course, it's really no risk at all given her lack of competition -- Clinton can show insiders, donors and activists (aka the only people paying any attention at this point) that this campaign is going to be different than the last one.
Add it up and there's really no reason for Clinton not to get into the presidential race shortly after the 2014 midterms conclude. Unless, of course, she decides not to run.