Sure. There are enough guys shuffling around the system that even in a net negative situation, people will get in to that base.
Example, let's say Dallas is a -3 this month but Baltimore, Atlanta, and Orlando are all plus four. 8 guys may leave Dallas for whatever reason (in reality usually many more leave every month in search of a better commute, better line, snowbirding, whatever) and 5 guys will get in.
Displacements occur, but they are pretty rare. You usually see them in ATL, MCO, and sometimes in DEN. SWA likes to shift manning around seasonal flying and those bases have the least number of guys willing to leave.
In order to be displaced, your base has to go net negative and there has to be not enough guys leaving to make up for the net negative. In other words, a senior guy bidding into a base cannot displace someone junior.
Being displaced blows if you are living there since there currently no right of return. It is in our new contract, but our ridiculous implementation schedule says it won't be a thing until next year. AirTran guys really got hosed by this with ATL during the merger. (Yes, I said it). Now any senior guy who wants to hold ATL can just bid in there and the displaced AirTran dude is SOL. Their flight attendants negotiated it but the pilot side did not.
It can be a good thing for a commuter since being displaced gets you either a paid move by the company or paid commuter hotels for a period of time. I know of guys who have purposely bid a base plug trying to get the paid move (unsuccessfully).