First, what Vanguard designates as Emerging Markets countries and what Morningstar designates as Emerging Markets are different, so one needs to be aware of that when reading any recommendation and when calculating one's own allocation to Emerging Markets.
Second, on Bogleheads perhaps lots of posts show 40%-50% EM, but that doesn't mean that many posters have 40%-50% EM. I think most folks over there recommend the market weight for EM.
Third, I will buy more EM when it drops significantly and sell when it goes back up all while holding a core position in it. The core position is the market weight of about 21% of international equities (Vanguard definintion of EM countries). I am currently slightly underweight with 20% EM after a recent sale. However, I overweight EM small caps. I note buys and sells in the market timing thread:
http://www.early-retirement.org/forums/f44/lol-s-market-timing-newsletter-57042-10.html
I wish to note that my int'l equities are about 30% of the portfolio (and 50% of equities), so 20% EM of 30% int'l means EM is about 6% of my portfolio. Furthermore, the OP stated "recommends 10-15% of international holding to emerging allocation." That would be a significant underweight to the market weight of 21% which I cannot believe Vanguard would recommend. Is there a link to the Vanguard article that one can read?
Fourth, since the low of the year on Feb 3rd, large-cap EM is up 10%, but US small-cap value US is up even more, and of course, everything is up.
So my conclusion is that EM is something to use, but one shouldn't have 40% to 50% of their international allocation in it. EM has been quite a drag on one's portfolio the past 3 years.