It's done to avoid crossing two strings (skipping one) from the F on the E-string and the Ab on the D-string. It flows better if you shift. That being said, it won't work well if you are not used to shifting and playing in third position.

It depends how you shift. Shifting correctly and accurately is an art and comes with time. As a matter of a fact, you can enhance the phrase with a nice shift. Crossing over two strings perfectly without being heard is also an art.
Fingerings are always suggestions. You should do what works for you.