dual classing TO a fighter

It's pretty standard to dual class from a fighter to a mage/cleric/thief/druid for the HP and THAC0 bonuses, but has anyone ever dual-classed from a one of those to a fighter (or cleric to ranger)? any creative advantages? as a mage, would level dependent spells (i.e. magic missile) increase with fighter levels? would you be able to attain grand mastery with your fighter levels?

i could see a swashbuckler/fighter or a totemic druid/fighter or a priest of lathandar/ranger?

also how does dual classing/multi-classing affect strongholds in BG2?