More and more people are choosing not to these days because it isn't seen as such a sin anymore to bear children out of wedlock. Not tieing the knot has got many advantages and not many disadvantages now.
If your married and split, the woman gets half no matter how much of it she may have contributed to.
If your married and split, the costs of divorce (not to mention the costs of the original marriage) can be huge especially if custody and lengthy court battles ensue.
The average marriage these days (UK) costs £15,000 - that's taking into consideration everything a marriage includes. (Info I got from a TV documentary)
Marriage is not necessary for committment, loyalty and love. It doesn't make these feelings any stronger than an unmarried couple would experience.
Forgive me for falling into the 'cheap' trap but it must be the student coming out in me!!
Unless it is done more for RELIGIOUS purposes (which it shouldn't - it should be mainly for love of your partner), it is a commercial event in my eyes with a hefty cost to whoever foots the bill.
One thing that has popped up in the news recently is the right to a partner's belongings after they've died if there is no marriage binding agreement that entitles the partner to them. One of the main disadvantages to non-married couples that may be resolved in the near future hopefully.