There is no real reason for a brand new player to enter open play straight away other than the desire to meet, greet and/or shoot other players. Everything you can do in open play you can also do in solo or private groups. The only difference is if you see other players and which players you get to meet, greet and/or shoot so the best thing for newbies is indeed to take a while in solo to get the feel of things then join open play.
Why you should join open straight away? Sidewinders are free, so you can experiment risk free as long as you aren't moving cargo. Your early days are when you should do all sorts of crazy stuff. Figure out the mechanics and learn how to interact with other players. When you have nothing to lose, you are risking nothing.
See that human viper over there? Yeah, go shoot that with your stock sidewinder 15 minutes after install. You aren't going to lose jack if you fail and it will be hilarious if you succeed. I took my own advice here when I started and killed a human viper with a stock sidewinder in my first hour or so of play. It helped that he was a bit dumb, probably someone who played solo until they could get a viper.
And that brings up the big draw back of solo early on. If you wait until you've established yourself a little, then you're going to suffer much bigger losses when you join open. It is likely to be a terribly frustrating experience. You will have only encountered AI and AI is hilariously dumb, as it is in nearly every game ever made. You will be terribly ill prepared for an encounter with a hostile human. You also can't get help if alone.
Yes, there are tons of smacktards in MMO's and MP games in general, but there are also plenty of people willing to take a new player under their wing. For my own part, I'm an aggressive player who will shoot just about anyone I see. But I'm also happy to take an aspiring outlaw as an apprentice if they merely ask nicely afterwards.
