<p>Actually, Princeton, Yale and several others all fill roughly half the seats from the early pool; Yale probably goes the farthest in this direction if you include early pool applicants who are deferred and admitted later. </p>
<p>At the larger Harvard and Stanford, 45% of the seats are filled from the early pool.</p>
<p>There is little to be gained, strategically, from applying to a school within this group that is not your first choice. At each, your chances of admission are substantially enhanced by applying early. Within this group, only Princeton has the negative of forcing you to accept if admitted, and forcing you to withdraw other pending applications. </p>
<p>So strategically, Princeton ED is the least attractive option if you merely want to gain admission to the best school possible, preserving your ability to choose among several places to whch you may have gained admission. Presumably, this is why far fewer apply ED to Princeton than SCEA to Stanford, Harvard and Yale.</p>