Greetings Zaebis,
I purchased a cell phone and think it is a great idea for most people too. Often time is short in Colombia, and the cell phone cuts down on most of the logisitical hassles.
I do not see any reason to throw the phone away after your trip. You can use it again on your next one, after you purchase some minutes again.
Overall I think the purchase of a cell phone in Colombia is definately the way to go! I don't know about not being able to call a different prefaced phone numbers. I do know the cost per minute varied from preface to preface, but I thought you could call all the other Colombian numbers. I could be wrong on that, so maybe somebody will correct me on that matter.
Fathertime!