Here's me going through some pros and cons of working on more than WiP (Work-in-Progress) at a time:
Pro: If you get stuck on one project, you can move on to the next for a little while, and so forth and so on.
Con: You might confuse elements, characters, etc. of one story with another.
Pro: Diversity keeps things interesting.
Con: Jumping from project to project doesn't encourage self-discipline.
In the end, I think it comes down to personal preference. A body's got to chose what works for them, not necessarily what works for someone else.
I'm curious as to what others do and what they think on the matter. Feel free to share and discuss!