SB is useless only when you have already an all round C or PF with high IS and high ID, then SB is not cost effective, the money you pay for it are too much and you dont have a significant amount in return (more wins, more arena revenue and more commercial income)
tunjevina is a great example as rakba said. Im guessing he has C and PF with high ID AND SB but the IS is just decent or even bad so the salary is acceptable.
just look at this match
(110727648) season 49 finals
opponent has 250K C AND PF salary and tunjevina has a pf with 48k and a C WITH 140k (thats 70k difference!!!) . Their sum on shot blocking (16) is astonishing at such high level play