i have two teams, one that i rushed TH lvl11 and the other that i slow played and it is still at TH lvl8. both teams are full epic, but different hero makeup and i can clear arena to deity. on the THlvl8 team i did not focus on the walls, i.e. still low level, but i have almost maxed all defenses. the main plus to a low level town hall is gold raiding and gold defense. outside of lords league i almost never get attacked, i can keep 3 million in gold and rarely have a shield up. i also get easier match up when raiding for gold and there are a lot more people with 100k + to loot. with my THlvl11 account i have to cycle through base after base looking for an easy match up, i.e. wont lose any heros, or a monster gold stash. the only down sides with the THlvl8 team is i can max level the hero alters, but these dont matter when attacking in BR or guild clash or lord league or raiding, i.e. you dont get the alter bonus if there is no alter. another downside is i cant upgrade my ancient relic high enough to learn allied winds, this skill is essential for major dps in BR and guild clash.
Final recommendation: Depends on your play type
casual players: keep a low TH level, arena is still the best way to build up a hero
2 hour + per day players: TH11 will allow you to max your heros alter, i still haven't gone above that because i dont want the added difficulty and i dont need to be a higher hero level at this time.
top level dudes: ya you need max the town hall to be the best at everything.