The word "incumbent" has several definitions, and in the political sense is generally used to denote current elected officials seeking re-election.
Not those who may have been appointed to fill out an unexpired term, instead of being chosen by voters.
Nor those seeking office for the first time.
Incumbent is also used to denote an obligation.
It is incumbent the media use the term correctly.
It is incumbent voters are aware of the voting records of current officials, to determine whether their own views are aligned with those of the candidate.
Having the (I) next to a name on the ballot is enough to swing undecided or uninformed voters, who may perceive that candidate as the most experienced or the least of other unknowns.
On official documents -- campaign materials, ballots and in the media -- the term incumbent should be used only next to the names of individuals seeking re-election in local, state or national elections.