That may be so. Where I live, in SF, they make no difference (there are seconds indicators on the crosswalks --they do not change at the pressing of the button). Nor does pressing them at a crosswalk in the stop phase change normal traffic light phase change.
I've never seen a button increase the crossing time, either. What I have seen, however, is that on a light that normally has, say, a 10s green with no crossing indicator, pressing the crossing button will extend the next green to something like 30s and display a crossing indicator.
I guess that's true in (some?) cities, but most places aren't cities. In most places, the crossing signal probably won't turn at all unless you press the button.
Here is some info: http://youarenotsosmart.com/2010/02/10/placebo-buttons/