Halbarad Dúnadan (profile of course from before the Hobbit rules) already specifically stated he could use a bow when holding the Banner of Arwen Evenstar - and that's a darn precious piece of cloth. And a big one too. He can simultaneously be equipped with a spear, and why not, do all that mounted on horseback too.
Having banner-wielding warriors use shields now is no oversight, it's the specific intention of the Hobbit rules. They left out that part to (I guess) streamline the game, just like they dropped the 'can't spear-support models on larger bases' rule. (And then compensated for the more streamlined nature by including those special strikes...
). A goblin spear-supporting anything large doesnt'make sense with those puny sticks they consider spears, but ah well, it's the rules. No more justification needed, unless it can be considered a loophole. Which it isn't.
As long as you don't pike-support and shoot in the same turn, there is ample time to temporarily drop the pike, sling the crossbow to the front and start reloading it. After all, an archer can also shoot in one turn and charge in the next, apparently having time to get his bow somewhere safe and draw a weapon. Gimli can fight with a two-handed axe, next turn chuck a throwing axe and then continue fighting with two one-handed axes. Having a crossbow and pike doesn't sound that strange - tricky to organise perhaps, but these
are the Fighting Uruk-hai. *rawr*