The piece of mind that an alarm offers is reason enough to have an alarm fitted. Unfortunately most people will not get an alarm until they've been burgled; a bit like closing the stable door after the horse has bolted.
Many people will say they have nothing worth stealing, but the intruder will not know that until they've looked around the property which can be very distressing for the owners. Having said that, most homes have videos, CD's, jewellery and, very important, items of sentimental value that money cannot replace (deceased relatives wedding rings, watches, video tapes etc.). It's also worth noting that many intruders break in when the owner is in the house so you have the personal safety aspect to take into account.
Very often the invasion of your privacy is far more distressing than having items stolen causing many people wanting to move home afterwards or having the anxiety of wondering if they've been broken into when the home is ever unoccupied.
Finally, you often have the inconvenience of re-securing the home where the intruder broke in (often more than one door or window will be damaged before an intruder gains access), completing insurance forms (knowing what was stolen and proving its value), taking time off work to meet tradesmen and assessors, paying any excess insurance, and facing the possibility that your insurance will insist that you get an alarm before they'll renew your insurance (and other insurance companies will take into account the fact you've been burgled before quoting so reducing your flexibility to shop around for the best price).
Statistically, once you've been burgled you are more likely to be burgled again.