Javelin attacks from the rear are definately not useless. I have make the most elite phalanx units rout with well timed javelins fromthe rear. You just have to wait until they are a little tired or their morale has dropped a little first.

One other reason that missile fire from the front against phalanx units does so little is that EB has the shield value off all phalanx units at 5 no matter the actual size of their shield. IIRC, shield values are doubled against missile attacks from the front/left front.