First the fallacies
1 Buicks have more nickel in there heads therefore don't need hard seats
      To start with nickel makes iron malleable not hard
2 Buick heads have hardened seats
      The only heads that Buick made that had hardened seats were 1975 and 1976
      Some Stage One heads in 1974 had hard seats installed at the factory
3 You can't put hard seats in Buick heads without going into the water jacket
      I have done hundreds of Stage 1 conversions 99% of which I installed hard seats and have
never went into the water jacket nor have I ever had one leak to my knowledge
4 You can run a lead additive
      For the most parts additives of any kind are never proportionally as good as when fuel or
oil for that matter is processed. I never recommend additives of any kind and look at most as
snake oil.
I suppose you may use it as a crutch but I have not seen a test even by those that make it that it
works only their claims that it does.  
Why Buicks didn't have hard seats
Until 1974 when federal guidelines dictated a phased in reduction of lead in highway gasoline the
manufacturers  had no need to have seats protected because of the current lead in fuel.
Since most of the 350, 400, 430 and 455 engines that we use were made prior to that they don't
have hard seats.
Types of hard seats
Alloy: The exhaust seat is cut out of the head and a false alloy seat is installed
Hardened: The exhaust seat is flame hardened by a controlled amount of heat applied to the seat
area by induction at the factory.
Why you have not had seat recession without hard seats
1 If the engine has never been apart the seats in the heads got somewhat work hardened over the     
years and they are protected to some extent by that.
2 If you have had it apart and done a valve job which removes the depth of that hardening and you
don't drive your car hard you might expect to have a reasonable amount of time before you have
seat recession but in time it will happen.
Why you need hard seats installed when you do a valve job
1 The more power you make the hotter the seats will get recession will occur
2 The harder you drive your car the hotter the seats will get and recession will occur
3 Sustained highway speeds will make the seats hotter and recession will occur
What is seat recession
When the engine is running as the exhaust valve starts to open the spent exhaust gas is like a blow
torch trying to burn the seat.
Lead would put a buffer on the seat that helped prevent seat recession
I tried to make this informative in a terms that was understandable

In conclusion the installation of hard seats is insurance that you don't have
to do a job a second time at more than twice the cost of the first one.