May the force be with you, you are a retarded idiot. Jk, but seriously there are more differences in races than just the classes they can be. Racial bonuses can be a very crucial part of your worth in some situations.

Let's say you want to play a rogue...a horde rogue.....ok. Options? Orc/Undead/Troll

Orcs are best known for their high stun resist (if you plan to take part in any player vs player [PVP] action, then you will find out this is a great thing to have)

Undead for their "Will of the Forsaken" being able to break free from fear effects and what-not (also great for pvp, and player vs environment)

Troll? eh....what's a troll :)

All I can say for sure, is that Alliance guilds will recruit any level dwarf priest, level 1, level 59, doesn't matter, at a fairly early level dwarf priests get "Fear Ward", pick a target, and he's immune to one fear for 30 seconds, luckily the cooldown for fear ward is 30 seconds ;)  THIS is phenomenal for PVE/End-game progression and will make guilds want to eat you up