Sandra,
If you intend to secure your luggage with a Padlock (rather than a Cable Tie or whatever), using the TSA-approved Locks is a good idea. If they decide to randomly search your luggage, the agents will be able to use a special key to open the Lock, and will hopefully re-lock it when they're finished. Some TSA padlocks have a small "flag" which changes colour if the lock is opened with the key, rather than the combination. If you use non-TSA locks and they decide to search your bag, they WILL cut the locks off.
Most of the small luggage locks are cheaply constructed and don't provide much security in terms of forcible attack. Despite the limitations, I always lock my checked bag and only once had a problem. I prefer to use a lock mostly to prevent "opportunistic theft" and to keep the bag closed during transit. On a flight from the U.K. to Ireland several years ago, the lock was cut off even though it was a TSA model. I'm assuming that agents in Europe may not all have access to the keys (or were too lazy to use them). They didn't close the zippers properly after the inspection, so consequently when I collected the bag off the carousel it was open. Fortunately I had everything well packed with internal cinch straps so nothing fell out.
As far as "spending money" for a TSA Lock, I wouldn't think that $8 would be a huge expense. Magellan's has a set of three TSA Locks for $13.