3 Reasons To ALWAYS Max Out Your Town Hall
1. Loot Penalty
Every time you go up another town hall you loose available loot. EXAMPLE: If a TH7 attacks a TH10 you will get 200% of the available loot so instead of getting 100K gold, you will get 200K gold
2. Less Time In The Next Town Hall
As I explained in 1, you will not get as much loot, so you have to raid more. If you raid more, your loosing more loot and time.
3. The Intimidation Factor
So lets be honest, if your farming at TH6 and you come across a maxed TH8 are you going to attack it? NO! You see pink wizard towers and its an automatic skip. You get intimidated and don't even want to try. So better defences means you keep more loot.
TL;DR Three reasons you should max your TH is because of the loot penalty, less time in your next TH, and the intimidation factor.