Having grown up in a very old school type education, I was much the same - if they're holding good weight, not losing grain and not having problems under saddle, then they're just fine!
But how many of us have horses that suffer in silence? I know a mare that was ridden to oozing blood from a saddle sore and never let her rider know - she's also never had her teeth checked in her entire 12 years and I bet you money she's just riddled with old scar tissue because she's to docile to complain.
We had the vet out today, mostly because we always have him out when the youngsters turn about 3 or 4 to pull any wolf teeth and give them a check. Zierra's first and only float was when she was 5 years old - and that was 6 years ago. She got her second one done today and I feel like a complete and total jackass because her mouth was riddled with old sores from catching her own tongue between the bit and her own razor tooth. But she was fat and sassy, not losing grain, and not acting any differently so she was fine!
From this day forth, I do solemnly swear I will dedicate myself to a minimum of an annual check for any and every horse I own. No horse should have to suffer in silence because they're too gentle to complain and we're too selfish and ignorant to want to part with $150 bucks a YEAR for their dental health.