It's purely about cost efficiency. If we take player with JS 20, JR 17, OD 17, IS 7, DR 19 and HA 19 salary is about 148k, but if you change IS to 20, salary is about 151k (salaries from BuzzerManager). IS is free for guards (salary and potential) so if team wants to be competitive and salary efficient, pure SG with only JR but no inside skills doesn't really make sense.