Weaponskill also determines your chance to hit (and hit well for that matter).
If at level 60 you go learna new weapon and attemp to hit a level 30 mob, there is a good chance it will miss. Similar to other skills, weapon skill (liek you said) scales by 5 for level.....so having a weapon skill of 150 for a level 30 mob is optimal (and above 150 is of course....more optimal??) Lets say you have a weapon skill of 150. You are fighting a mob that is level 34. The optimum weapon skill for that mob would then be 170 (34x5). Since you are only at level 150, you have a decent chance to miss the target all together or get "Glancing Blows" which are attcks that only do a portion of the intended damage.
+ Weaponskill becomes hugely valuable when you reach towards endgame battles. Fighting mobs that are ?? means you WILL miss. But, if you have a weapon skill of 315, 330, etc. you will miss less. For normal leveling, it (in my opinion) is not nearly as important as other stats, as you probably won't be facing too many encounters with mobs that are THAT much higher than you. Maybe this is different for paladins.
Defense: I will admit I really don't know that much about it. BUT, similar to the above post, it does scale 5 per level, and the more of it you have, the greater your damage reduction will be.
From a sticky in the Statistics and Formula section:
Increases the chance of being missed by an attack.
Increases the chance to dodge, parry, and block.
Decreases the chance of being affected by a critical hit.
Decreases the chance of being affected by a "crushing blow". Creatures that are higher level than your character can land crushing blows that deal increased melee damage. The chance of a crushing blow increases as the level difference between you and the opposing creature increases. Players never deal "crushing blows", only creatures.
SOURCE - Along with info on many other skills