Even though trimming won't make your hair grow, you still don't need to neglect those ends. Split ends split up the shaft at an angle until they can't go any further and then break off. (I have a hard time believing that it's possible for a hair to split to the root at an angle, but I'm no scientist or professional.) Anyhow, where the hair has split and broken off will become thin. To prevent this, you should trim off any split ends. Some people do this by regularly "dusting" their ends, which is fancy for searching for splits in ther hair and only cutting those off.
How often should you trim or dust your hair? There is no definite answer to that. My beautician used to trim my hair every eight weeks after each relaxer. She trimmed...well really cut at least one inch off each time. Hair normally grows one inch in that amount of time! As a result my hair stayed at the same length for years. You don't have to trim that often or that much! There is no set rule for how often you need to trim. I try to trim once every six months or so because protective styling has really kept my ends in tact.