My guild raided into Naxx40 back in the day. One of our offtanks was a Paladin. There were a few times where he MT'd MC for us earlier on. We also had a Balance Druid and a Ret Pally who I remember putting up competitive numbers. They wouldn't top the meter, but they'd be in the top 10 dps most of the time.
Of course, the norm was if you could tank, you tanked, or if you could heal, you healed. If you had both, you healed. That doesn't mean that there weren't players that did well in other specs... they just weren't meant to.