i could be wrong, but my immediate feeling is that "be"
can substitute for "is" when the "will" is omitted.

"he'll be down the pub, most likely"
means the same as
"he's down the pub, most likely"
and
"he be down the pub, most likely"

what francais posted was a subjunctive (if i be correct), so perhaps omitting the "will" inflection gives a more potential rather than definite feel to the phrase.