Janet has given Pippi the idea that libraries are very dangerous places, and Library Wenches are like the coolest people to her next to Doctors and Policemen.

So if the Library Wenches and the rest of the B&E group are going to seperate, Pippi would volunteer to stay with them and be a lookout/body guard while they have their noses in the books.

In cannon she can be like all Solid Snake Style stealthy.

She just doesn't bother most of the time.