Terri pretty much covers it. This came about last year when one of the leading accomodation only operators went bust. It wasn't til after that ABTA realised they weren't holding an ATOL licence for accomodation only. Therefore customers who had booked individual components couldn't claim their money back on the accomodation. Which was almost always the most expensive part of the holiday. The moral really is to check when you book seperate components that each company holds an ATOL and to check your own insurance cover to see if you will get refunded should a part of your holiday go down the pan. I.E. if the airline goes bust will you get a refund on your villa because there are no other available flights to get you out there for that week?
By booking all in one package you are covered completely if for instance the hotel burns down (happened 2 days ago to some clients of mine). The tour op is obliged to either refund you in full or find you a suitable alternative. This goes a little way to understanding why most packages (but not all) work out more expensive than seperate components.
You get what you pay for, unfortunately most people don't understand they need all the protection until its too late.