How important is memorizing chess openings in casual play?
Its far better to thoroughly understand a small number of openings than it is to mindlessly memorize a large number of openings.  Frequently, if you know why a set of moves is considered optimal, then you're in a better position to adapt once the board gets "off the opening".  However, if you have a wide set of openings memorized, you may find that while you are moving down your 'optimal opening', ready to transition to a strong midgame, your opponent may have other plans, and make a move that takes him off your chosen opening.  

Now frequently, there's a good reason why that move is not covered by your opening chart;  that move may be terrible.  But if you don't understand why that move is terrible, then you may not be able to take advantage of it, and if the other player improvises better/faster than you, you may find yourself at a disadvantage despite your 'superior' play to this point in time.

Don't let that happen to you.