I was talking about early stage entrepreneurs. Working on multiple ideas once you are successful or have enough experience sounds logical but initially that might be a hurdle.
I think it varies from case to case. I wouldn't generalize it.There have been cases where things which started as a side project at a startup have become their main product.
At the same time there have been cases where startups didn't take off because of focusing on too many things.
So I think it's always better to work on complimentary ideas which might blend well together. Working on totally unrelated ideas at the same time would spoil the show. Again, I might be totally wrong too.
Check this article for examples. Really good one http://ryanhoover.me/post/98567029388/startup-side-projects