Full Bijuu mode isn't too useful for combat, so the only thing I can see it doing here is clearing out the forest to prevent Jiraiya from hiding to prepare Sage Mode. However, considering you have Jiraiya starting in Sage Mode anyway in the scenario where Bee can turn full Hachibi, there is no point to it. In the scenario with the toads summoned, the only real purpose of Bijuu Mode is to nuke the toads. V1 and V2 are much better for small scale combat since Bee gets to retain his speed and mobility while simultaneously not making himself a giant target.
The forest location helps Jiraiya with preparation of Sage Mode, and full knowledge means Jiraiya knows it is suicide to engage Bee in close quarters. Assuming Jiraiya manages to enter into Sage Mode, Killer Bee is still faster, stronger, and possesses far better Taijutsu skill. Bringing out V1 and V2 form furthers the already existing gap in those skills.
Frog Song, which is usually Jiraiya's way out against stronger opponents, falls flat on its face here thanks to the partner method. Jiraiya's only real hope is nailing Killer Bee with a giant Rasengan or Goemon, but that in itself will be difficult due to the speed gap. It's also questionable how much damage those techniques will do to Bee in his cloaked forms. If Bee has Samehada, he can probably soak up most of the attack with it anyway.
