There's no blanket reason to avoid them. The Managed Payout funds are simply balanced funds that pay out a predetermined amount each month. If they have an allocation that is interesting to you, they make an extremely easy one-fund solution.
As for not eating into principle, that may happen regardless of whether you own one of these funds or any other fund(s). If you need to make a withdrawal from a fund that doesn't offer a large dividend payout or have any capital gain to speak of, how do you avoid it? This likely happened to many people in 2008 who had any significant stock exposure. Many people likely cut back on withdrawals. With the Managed Payout funds you could also opt to not spend the entire distributions you get and reinvest some of them.
Personally, I think these funds are fabulous and have owned the middle one, Growth and Distribution (VGPDX) for several years and couldn't be happier. Its nominal allocation is pretty much exactly what I would have in individual funds if this fund wasn't available, and the constant-percentage payouts (averaged over three years) is, in my opinion, far superior to the often discussed constant-dollar method.