Agnostic Monk Speaks---
Linda, Linda, Linda! LOL to Col. :)
But actually that is a good question. I would lean towards the drivers license reason for her inclusion (and not because I'm Linda's official P.R. person, more because it sounds like the simplest explanation). There may have been some other folks at the ranch with drivers licenses but maybe they were unavailable for Cielo due to various reasons (wasn't Patricia Krenwinkle trying to sleep off an acid trip that evening when Charlie woke her? Maybe others had been tripping that day and judged too messed up for the mission?)
Being stopped by cops with a licensed driver in the car would be far better than not having a licensed driver in the car (no matter who was driving). And they were of the mindset that if they were stopped they could throw knives and a gun out a passenger window without a cop noticing, most likely they figured switching drivers could be done without the cop noticing, prior to his initial approach towards the car.
But who knows, maybe there are deeper reasons. Weren't there rumors that Tex and Linda had been dealing together?
We could always go by Sandra Good's reason as stated in the Manson documentary (with rifle in hand). Something about Linda had been to the Tate house before so they all got in the car and Linda drove them to the Tate house?
So if you and I are driving drunk today to Magic Mountain Amusement park and they pull me over and I don't have a license I can say "Don't bother me, Mr. Man, because da Monk here has a license."? I don't think so. And that was just drunk driving. Can you imagine Tex, stoned out of his face on speed, saying, hey, don't hassle me, the tiny blonde in the back has a license? Nope, me neither.