It should skip every other label, but it doesn't (version 2.1.0).
I believe the fix is to change the following 2 lines in pDraw.class.php:

$LabelSkip = isset($Format["LabelSkip"]) ? $Format["LabelSkip"] : 1;
should be:
$LabelSkip = isset($Format["LabelSkip"]) ? $Format["LabelSkip"] : 0;

AND

if ( $LabelingMethod == LABELING_ALL && $LabelSkip == 1 ) { return(TRUE); }
should be:
if ( $LabelingMethod == LABELING_ALL && $LabelSkip == 0 ) { return(TRUE); }

Guillaume

Good morning Guillaume,

You've found a bug and the correct fix ;o)

Thank you very much. I've made the change in the next minor version that will be published soon.

Kind regards,
JD.