On many occasions, Chakotay is asked to take the helm, tactical and other posts when another officer isn't readily available for it, yet we've never been told anything about his background that leads us to believe he'd be the ideal person to take over as a relief anything; he's a social scientist and as such would have been wearing a blue uniform pre-command.
He was also an experienced pilot, it Future's End he tells B'Elanna about his qualifications, so him covering for Paris makes sense. Plus tactical controls would be laid out in a manner to select weapons, firing pattern and then press shoot button. Ensign Taitt (a science officer) manned tactical in Descent easily, whilst in Generations Troi took the helm. Starfleet officers will be trained to cover posts, in case of dire emergency and they're the last one left on the Bridge, it won't be their speciality, but they'll at least know how to press the buttons.
The main reasons the senior officers take over at other posts are that: (a) anyone who could have filled in has likely been needlessly killed off at some point before, and/or (b) its cheaper to move them to a console or on an away mission than it is a guest character or extra who'd have to be paid to speak.
Commander Austin Harris, First Officer, Deep Space Nine (by FltCpt. Bossco)
8.01 - Darkest Before Dawn
(Chapter 8 added, 12/09/2015)