Having the G17 at the start sets the condition for the first cut. It lets the operator know the current setting of the plane. If, by chance someone had reset the plane to G18, this will make sure it goes back into G17 before the first cut. In general it's safer. If the machine is already in G17 and you set it to G17, nothing happens. It doesn't take longer to run. It has zero effect on the performance of the program.
Depending on how it was that was removed, it's now possible, that if your first cut needs to be in the G18 plane, that might not show up (depending on how the change was made). Now you'll be in trouble.
Why is it so important to turn off something that has no effect on the performance of you part?
It's the same as taking the "P" off of the shifter display in your car because ... "when I get in my car, I already know it's in Park".
Modifying a posts that's not technically broke can cause problems further down the road, depending on who and how they made the changes. Someday you may update that post from X5 to X6 and the particular way the change was made, may cause something else to stop working. I cant tell you how may posts I've had to edit because some one "fixed" them.
Just some things to consider when modifying your posts.
Mike Mattera
Tips For Manufacturing Training CD's, DVD's for Mastercam, SolidWorks, Inventor, G-Code Training & More
http://www.tipsforcadcam.com