MrZeddaPiras
[insert something clever]
Or if you don't want to spend a lot of time on it, you could just say that Blagothkus is grateful to the PCs for their help and that when the time comes, the giants will join the fight against Tiamat. You could even have him say this before the PCs leave the flying castle, if they haven't already done so.
As for Blagothkus' motivations, one small addition satisfied my group. I had Blagothkus state that it was better when the giants were united against the dragons because now, they fight amongst themselves instead.
Yea, that's what I planning to do. The fact that there were some clarifications about the giants that were cut from the book explains a lot.