Lining is intended to protect the gun finish from holster wear. Not sure if lined vs unlined makes much of difference for most people drawing if it is properly broken in. I do like leather for most purposes EXCEPT for tactical work. Many professionals who use a holster on the job have gone to a kydex or similar type holster. I personally like an open top (no thumb break) kydex when wearing soft clothes mainly because its so incredibly fast.
Not being a competition shooter, my understanding is the same--the lining saves the finish--but with the caveat that most competition SASS rigs are lined, and I assume those guys know a lot more about it than me.