Soon as Jaheira hit lvl 20 fighter and lvl 15 druid in BG2:TOB she got: 5 lvl 7 spells, 4 lvl 6 spells, 3 lvl 5 spells, and 1 lvl 4 spell.

Needless to say, she suddenly got very, very useful.

yeah, it's a pain in the ass to gather all that xp though since druid needs 1,5mil xp from 14->15lvl and as multiclass she needs twice as much.

i think it's made that way so druids wouldn't be too powerfull in amn (the exp cap in amn is 2,95mil when tob raises that to 8mil) as druids wont get 7lvl spells at all even when single class

Isn't the Arch Druid supposed to be the only Druid over lvl 14 in the Forgotten Realms, with regional leaders being lvl 14?
AD&D geeks, correct me if I'm wrong.

The real reason is that AD&D druid progression is just bizarre as hell, and they didn't feel prepared to implement it in base BG2, which is why they didn't allow druids to get to level 15 in SoA. In ToB, they just said "screw it" and gave them a few (but not all) of the powers, but there was still an enormous XP gap between 14 and 15 that doesn't exist in the PnP game. Personally, I messed around with the XP and spell progression tables in order to smooth out the transition from SoA levels to ToB levels, which makes life a lot easier for multiclass druids but they lose a few HLAs. (The only repeatable HLAs are elemental transformations, though, so it's not really a big loss.)

There can only be nine 12th level druids ("druids") in a particular region. There can only be three 13th level druids (archdruids) in a particular region. There can only be one 14th level druid (great druid) in a particular region. There can only be one 15th level druid (grand druid) on the ENTIRE PLANET, but he gets a bunch of extra bonus powers. However, once getting to level 16 he steps down as grand druid and can continue gaining experience normally as a heirophant, though he loses a couple of the grand druid powers. The entire system is very strange since you have to either wait for NPCs to retire or you have to kill them, and it is often ignored/altered by DMs. It's doubly strange because advancement in the church that isn't really a church ends up being substantially more political than the actual churches. Some official settings like Dark Sun disregard it entirely and have their own (more rational) system.