As this is a forum for the Baldur's Gate series, not just Baldur's Gate II, the first thing I want to mention in favor of multi-classes is, if you play a multi-class character in BG1 or Tutu, they are mostly only one level behind single-class characters, have both of their class skills available for the whole time. And if you want to dual at a higher level in Baldur's Gate I, it's a long wait for having your first class' skills back.
In Baldur's Gate II, dualing becomes a more viable option as you get the first 7-9 levels for free, depending on starting class. A long part of the wait is already over as your character already advanced through most of the starting levels in the Swordcoast, before waking up in Irenicus' Dungeon.
An advantage of multi-class characters in Baldur's Gate II is they keep advancing in their first class. This means, as mentioned, HLA's for both classes, but also, for example, Thac0 that keeps advancing at a higher pace for multi Fighter/Mages then they do for dual Fighter>Mages.
If you play the whole trilogy with the same character and decide to dual-class, you either play 2 kind of characters if you dual late (9 or higher), playing the first game with a different class than the second) or you dual early, but then the advantage of the first class in BG2 would mostly be a wider range of proficiencies and armour allowances, as there's few advancement in skills of the first class, compared to the high levels of the second class in BG2.
So much depends on if you just play BG2, just play BG1 or take your character through the whole saga.
Also much depends, as mentioned above, on the combination of classes you want to use.
There's a very wide range of class combinations. Just some I'm familiar with:
The multi ranger/cleric is a powerfull combo, even in the end-chapters of SOA. Good Thac0, good buffs, reasonable level of both cleric and druid spells. And a Ranger dualed to cleric is not an option if you want to play by the book. If you tweak the rules, allowing dual-classed Rangers, at least Thac0 will be a disadvantage of dual-classed Ranger>Clerics.
I'm familiar with both the fighter-mage and fighter>mage.
The fighter-mage is basically a fighter that makes use of arcane spells to improve his protections, using his spells for buffing himself, much like a Blade does, though with a different advancement curve.
The fighter dualed to mage is mostly a mage that has better hitpoints, Thac0, can use all weapons. But you will use him much more like a mage that is less vulnerable if he takes damage, as there's a wider pool of hit points. In the beginning, he can be played like a fighter-mage, but (I'm not very advanced yet with my fighter>mage but this is my projection) in the end he will be mostly casting spells, hardly fighting at all.
The thief/mage finally, like Jan for instance, will become a better thief as the game advances (and there's enough thief points to spend throughout the levels to make multi-class a more prefarable option then single class, taking another class 'for free' on the ride and still being skilled enough in stealth, locks etc.). A thief>mage for instance, like Imoen, will have to pick some skills to become good in and leave the rest at unusable low levels (Imoen is horrible at stealth, for instance, has to rely on invisibilty potions or spells for that) or rely on potions of master thievery, perception and items a lot. Finally the Thief dualed to something else doesn't become better at backstabbling, the multi does.
As to dualing to thief, your backstab does get better, but if you advance from a spellcasting class, you'll never be able to cast high level spells. Fighter is the most viable option. But then, there's possible some advantages to mention to fighter/thieves, but my post is long enough by now.