How come? Even if you don't see him as a top line center, a #2C is usually still a part of the core when laying out a template for building a team. Especially if he's in the 21-40 range leaguewide, which is how I would rank Dubois at his best.
What is a core? I'd say it's your #1 goalie, two of your best defencemen, two of your best centermen + one or two other forwards. This may vary, of course. Some teams have won having more of a journeyman types in some of those roles (goalie, #2C, for example), but the general idea is sound I think. Now, if you want to make a distinction for a micro core of say, 2-3 players, then yeah, Dubois is likely not that, but I doubt anyone is advocating paying the price equal to a top-3 player on a contending team.