Maybe, and maybe not.
Also, such a simulcast may extend to any potential championship-clinching games, not just a possible Game 7.
Maybe:
It would give fans in outlying areas a chance to listen to the game on radio.
I'm not 100% sure, but I think that Dial Global is doing a national radio broadcast of the Finals, and if that's the case, their deal with the NHL may force regular Boston Bruins' radio affiliates outside of Boston to carry the Dial Global broadcast instead of the local WBZ-98.5 broadcast (I know that's the case with Dial Global's broadcast of the NFL Super Bowl).
Such a scenario would favor a simulcast since WBZ-1030, being in Boston, would be the only other station that could carry the local broadcast, and the only way fans in other parts of New England would be able to hear Dave Goucher and Bob Beers' call of a potential championship-clinching victory.
Maybe Not:
In the 2010/2011 season, the Bruins' radio network had only two or three affiliates outside of Boston. I believe now, there are about 25 stations in other parts of New England that carry the games. The big WBZ-AM signal might not be "as needed" as much as it was two years ago, especially if these outlying stations are able to carry the local Boston broadcast.
One other problem is this: Should CBS Radio Boston decide to simulcast Saturday's (June 22nd) Game 5 on WBZ-AM (if the team wins Game 4 tomorrow, June 19th), it would displace several hours of paid programming WBZ-AM has on Saturday nights (Other than the "CBS Weekend Roundup", the hours between 6 and Midnight on Saturdays are filled with paid programming; in fact, I think even Kim Kommando is a paid program).
My view: There is, in my opinion, a 50-50 chance that WBZ-AM will simulcast any potential Bruins' Stanley Cup-clinching games.