In my mind this does not fall into the category of "professional behavior" but is just plain ol' good behavior. Here is what the folks at the Culture and Manners Institute have to say about saving seats:
In the crowded gate area in the airline terminal there are few empty seats. A tiny woman with dark hair places her laptop on one seat with her bag in front of it and leaves an empty seat next to it where she plans to sit. Then she moves to another set of seats and sits and chats with two co-workers. An older man comes along toting his luggage and makes for the empty seat. She stops him and says, “Oh no – that’s mine.” The man pauses and looks at her sitting in the other seat, then shuffles on.
No one should be able to hog three seats in a crowded gate.
In adult life, there is no such thing as “I called it!” There is no such thing as “this seat is saved” when the seating area is filled, when the train or bus is filled, when the theater is filled. If your friend or kin has not arrived, that is not the problem of the person who is there and looking for a place to sit down. Your bag, no matter what kind of day it's had in the underbelly of the airplane or going through security, does not warrant a seat of its own.
Make room for others. Place bags on the floor or on your lap. Give up your own seat for those who are elderly, frail, pregnant, juggling small children or anyone who looks like they could really use it. Rather than say, "Do you want this seat?" say, "Please take this seat."