I don't have to, because it's just a pug.
Then there's the long list of don't have tos.
I don't have to:
Perform at max capacity
Use my buffing spells
Use my cooldowns
Enchant my gear
Gem my gear
Reforge my gear
Run in an optimal spec
Know the content
Run in a spec that's suitable for my role at all
Disrobe my PvP gear
Wear gear that is suitable for my class, spec or role at all.
Pay attention during fights
Be physically present at my computer during fights
Do you see a trend?
Remove items from the list that are clearly absurd. If you have more than the three top most left for a five man or any at all for a proper raid, you're not casual, you're just bad.