Besides the reasons already mentioned, It's generaly recomended to replace used valvetrain components to minimize wear on the parts as well.
In otherwords, new parts should break in together so they will last long... not with old worn parts with 75k miles on them...
Pushrod wear surfaces are minimal, but they are small and under a lot of pressure.
New cam lobes for example, running on worn lifters would tend to wear faster by comparison.
In the end you'll be glad if you do it right, rather than have saved a few bucks and have problems.
P.S. Dont re-use the TTY rocker bolts either.
