The extra requests are multiplied by the number of behavior-bound elements, which can make a Very Perceptible Difference on pages with many bound elements (think 100 or more). In this case, the page can load very slowly, showing the message "xxx Items remaining" (where xxx counts down to zero) in IE's status bar.
Fortunately, this situation is been very rare in my experience (only once was it slow enough to need an alternate approach).
TEXTAREA {overflow-y:expression(this.maxlength ? 'scroll' : 'auto');}