I didn't try multiplayer until just a couple of weeks ago, but I would argue FOR it.
I love SP, have 100%, been through it twice and will do again.
Arguments against MP are numerous, mostly the annoying 12 year olds, killing you as you spawn etc, but you can go passive and change the game. I found it a bit slow to start in MP, robbing shops for money, trying to rank up and next to nothing available to you while the 12 year olds all have supercars, fully modded with RPGs and the like.
This can make you feel 'alone' as somebody pointed out earlier, but if that's the case you probably are alone. I was at this stage.
Then it all got better. There I was, robbing a store when I ran out to find my getaway car had been nicked. I ran as best I could from the police, trying to make it to a bridge before the chopper saw me when another player pulled up & beeped, I got in, he got away from the law & invited me to his party so we could chat.
From there he's been really helpful. I joined his crew, we did missions and survivals to rank up, his friend did up a few hot motors for me to sell and I'm up and running.
Now, we're in a bike crew with proper heirarchy, a mix of ranks and abilities, but we're always on and having a really good time on it. We're beginning to get interest from other bike crews in a bit of 'friendly' rivalry which can be done in private sessions.
If you feel 'alone' or bored with GTAO, look at the GTAO forum's threads on meets / crews etc. Get some like minded players with you and these 12 year olds won't be a problem for long. I can see how this can be less than you hoped for if you're on your own, but I fail to see how anyone can give up on it. For every problem, there's a solution.
Okay - hackers being the exception.
BTW Re: crews. i first found a large open crew on social club and joined them. I've still not seen one member online. Small crews of friends is the only way. When I say friends, I've never met any of them, but I now spend every night with them.
ONLINE IS OUTSTANDING DESPITE IT'S FLAWS