Changing classes has a bunch of virtues and not a single flaw IF you do it always before or at 8th level.If you take a character to 5th level (Fairy Mage/Thief/Bard for example, who pretty much never needs to worry about stats). 50 times, before you decide to settle in as a Bard. By the time you reach 9th level, the guys in your party that didn't switch classes will be 10th level.
You'll have all the skills maxxed, know all the spells, have a lot more HPs (+1 50 times), a lot more stamina, a lot more mana, and that 1 level difference will not matter at all when you hit the things where it matters, like Gorrors.
The key is to find the classes/races that work best together.
Elven Valkyrie/Ranger, easy to do multiple times back and forth.
Gnome Valkyrie/Lord, easy
Elven Mage/Priest, easy (end as Bishop even)
Faiery Bard/Mage/Thief, easy
There's a couple more too, less easy (A Ninja/Samurai combo, and a Monk/Ninja combo).
All in all, it's BEST if you multi-class a lot, especially if you picked the best race and class-combo to do it repeatedly. There are no negatives, at all. Only positives, as long as you keep it all before 8th level (5th level is best).