To be more precise, Rangers can cast Druid spells but not Cleric spells. If you were to solo a Ranger in BG1, you
might reach Level 12 and be able to cast
one 3rd Level spell. But the experience cap in TotSC will limit you to 8th Level and the ability to cast one 1st Level spell.
I enjoy using Priest spells. Doom, Armor of Faith, Barkskin, and Resist Fire/Cold can give you an edge in combat. Call Lightning is a good offensive spell. Remove Fear and Dispel Magic can help restore your own party members if they become debilitated by enemy spells. Don't tell me that never happens.
The main drawback to equipping a Ranger with heavy armor is that you can't use stealth. You might as well be playing a Fighter instead of a Ranger if you don't use him as a scout or let him cast spells. But if the NPCs you like are Rangers instead of Fighters (Minsc and Kivan come to mind), then by all means play them as Fighters if that's what you prefer.
Rangers are more fun to play
as Rangers in BG2: Shadows of Amn because their special abilities are a lot more useful at higher level. Kits, armor, weapons, and other items also make a difference. It's too bad that Kivan doesn't appear in BG2 (although if he did, there's a good chance that, like a lot of other NPCs from BG1, he couldn't join your party and something terrible would happen to him right after you met him).
Ranger spells? 
Linear Mode
