As someone working alone, I find that not being able to bounce ideas off of a parter is not that much of a problem once you have users. You can evaluate ideas based on what users are saying or doing, or just try them out and see what happens. The real disadvantage is that it just isn't as much fun as working together.
But there are two categories of ideas: features to release to users, and decisions about how to write/structure code. Of course users are the best people to bounce features off of (although an initial 'partner filter' is often valuable), but they most likely have nothing to say about actually writing code, or for that matter almost any other business decision.
Bouncing code-related ideas off each other may be important for some. I'm a good enough coder that if I had someone to bounce ideas off of, I would be doing it more for fun than for any tangible business advantage. Not that my code is perfect, but I'm experienced enough now not to create any messes that will be a nightmare to clean up.
It's easy to find yourself justifying bad decisions that were made for the wrong reasons. A good partner can be a great reality check, in a way no one else could. This is probably the biggest single advantage for me because I can be dangerously stubborn on occasion.
If you work alone you can never be sure you don't fall foul of the Dunning-Kruger effect http://en.wikipedia.org/wiki/Dunning-Kruger_effect :-) (This was on reddit today which is how I know about it!)
Seriously though. I basically code alone for 8 hours a day but I bounce my ideas of off anyone that will listen and I freely give up half my coding time to do this when the opportunity is there. There is nothing like a second opinion.
I think the real disadvantage of working alone is that you have no one to bounce ideas off.