Opinion is divided on this one. I personally see my betta interacting with the other fish in the tank quite a lot - so I like the idea that he has a bit of fishy stimulation on his life. Some say that they are fine on their own and others still say that they are fine on their own as long as you give them some time each day playing about with a mirror to give them some extra stimulation.
On top of that, there are those few rouge bettas who try to beat up everyone else in their tank so they definitely need to be on their own lol - but by all accounts these bettas are far and few between.