Been out in an SCA war (some of you know what that was, and those that don't,
just know I was out of range of the internet).
I think that Roäc expects correctly that should Thorin call for Dain's army,
they will come expecting a fight, and if they encounter besieging forces, there
will be a fight, instead of parley. This means that a lot of Dwarves (as well
as others) are going to die, but nonetheless Dain's army will be unlikely to be
able to destroy the besieging forces. As a result, if and when a truce is
reached, there will be little gain for the Dwarves, and a lot of dead - even if
there is peace settled eventually.
As for Thorin's forces, well, I think that Thorin recognized correctly that you
do not fight Dragons with armies. Getting rid of a Dragon is a hero's business,
and sheer numbers aren't so useful. Now that Smaug is out of the way and
Thorin's hall is in danger of being taken by non-Dwarves again, and an army
might help, Dain can
come.
