i think this is a great question. i'm subbed for some serious replies. i've heard the same thing about it raising estrogen and about animal proteins being higher quality. but then i've heard that that's just bs, and there's also this chart on Wikipedia
PDCAAS value of protein
A PDCAAS value of 1 is the highest, and 0 the lowest. The table shows the ratings of selected foods.  
1.00 casein (milk protein)
1.00 egg white
1.00 soy protein
1.00 whey (milk protein)
0.75 black beans
0.70 Other legumes
0.59 cereals and derivatives
0.42 whole wheat
soy protein absorbs at a higher rate than beef?
what's most striking, though, is the low absorption of peanuts. so if 1 serving of peanuts contains 7g of protein, your body only gets about 3.5g of that?
yeah, so i'm not sure what to think either.