Remember that until the 1980's teams used a four man starting rotation and in the 60's and 70's generally only carried maybe 10-11 pitchers on the 25 man roster instead of 13. Pitchers finished what they started much more often so there wasn't the need for all the spot or specialty relievers. The top pitchers threw more than 300 innings, sometimes as many as 340 or so.
I still do not feel there were as many arm injuries in the 1960's as there now but I can't numerically prove it. Today, many pitchers are like cannon fodder with the massive numbers of Tommy John surgeries.
i agree with 3bagger and others who talked about specialization in pitchers. so few complete games now. i remember when knuckleballer wilbur wood threw in both games of dh with the whitesox. also go to showcases today and the scouts/coaches are there w/ radars. showcase formats in many cases favor the pitcher; for the patient hitter a 1-1 count to start an ab is a big disadvantage. pitchers not too recently were the best athletes and 2-way was common. dwight gooden was one pitcher who liked to hit carlos zambrano too.