Michael,

"provided you understand"

Unfortunately most people writing software don't fully understand and will probably get in a mess sooner or later if they call functions from ISRs. As the original poster couldn't work out why he(she?) was receiving the error message I'd say it's likely that he/she also won't be able to call functions from an ISR safely.

I do agree with your comments, particularly with your rebellion against the usual mantra "Keep ISRs short and sweet". What is appropriate in an ISR is entirely down to the individual application.