It is an important safety issue, but there does not seem to be any consistent industry-wide standard of application and enforcement. Some airlines ensure suitable occupants, others leave the exit rows empty whenever possible and others simply don't give a t0ss.
I was recently on a flight where the exit row was occupied by a person so large as to require a seatbelt extension. The load was about 40%. When I discreetly (out of earshot) questioned the CC about the policy on that airline, I received a dirty look and was essentially told to fork off.
Days later on the same airline I was in the exit row and got a detailed and clear briefing from the lead FA. Go figure.
(No names, but think maple leaf.)
[ 18 August 2001: Message edited by: PaperTiger ]