I also believe, that to some extent, it's very beneficial to have a big name in the field.
However, knowledgeable committees are also aware of what kinds of LORs these people write. Perhaps, for example, Professor X always writes glowing letters. In this instance, the 'big name' might not be the best option. But who's to know?
2 of my 4 LORs were from 'big names.' It doesn't get any better if they are respected and can write you a solid letter.
That being said, I'll take glowing, honest letters (from lesser known profs) over a mediocre letter from big names any day of the week.