Exactly as LaserLordz said, at that time it was good
enough.
But the AMX-13 were discarded in the 70's by India and Israel, both
of whom have used them in combat and found them to be completely
worthless. The small gun calibre problem was improved upon with a
bigger gun, 90mm or even 105mm. But the fact that it couldn't fire
on the move by 1973 was already considered dire. The other thing
that could not be improved upon was its thin armour, resulting in
loss of precious tank crews during combat.
The original masters of upgrading - the Israelis - upgraded the gun
to 90mm but couldn't do anything for the armour, so they gave it
up. (The upgraded M4 Sherman with 105mm gun was considered a better
choice over the AMX-13 and served them longer.)
So in a sense it was a "bluff" for, as impressive as 300 of these
tanks were in 1970's, they were already obsolete by then. (However,
it was a good bluff as in those days, Malaysia had only some
armoured cars and Indonesia some PT-76.)
So if they were obsolete by 1973, what are they worth in today
2006?