I think it's like giveing the hoplite the ability to throw his spear(and to continue fighting with a Xiphos)
or to give any unit with a massive(not those wicker thingies) shield an alternative AP attack

afterall every javelin can be used as a shot(and weak) spear or in simple terms a pointed stick. The Pilum is one of the best javelins when used as a javelin, but afaik less usefull as a spear than most other(simpler) javelins.
the most realisic way to implement this would be a third fighting mode with a mount bonus melee and limited by number of projectiles(if you use a pilum as spear it bends aswell thus being useless afterwards). as It is neither possible nor utterly effective I think what we have in EB 1 atm is most appropriate.