Speaking in parables

Mark
Mark 4:33-34 is curious:
33With many similar parables Jesus spoke the word to them, as much as they could understand. 34He did not say anything to them without using a parable. But when he was alone with his own disciples, he explained everything.
Why would Jesus specifically use parables?
Gill suggests that Jesus spoke in parables because the crowd would have up and killed him directly if he hadn't cloaked his message.  William Barclay, another theologian, suggests that Jesus spoke in parables to ensure that only the devoted followers would stick around to truly find out what he was all about.  My take is somewhat along the lines of Barclay's, that they wouldn't have accepted his message if he had just outright said it.  So, parables make some sense!  They require that the listener actually think about what was being said and work out the message that is hidden within, rather than taking the message at face value and probably forgetting about it soon after

Here is a link to sermon on Mark 4:33-34 by Charles Spurgeon (click here).