Cash was a solid player, but the Pirates had Rennie Stennett and Willie Randolph waiting in the wings so the deal made sense at the time. Brett went 22-14 over the next two seasons for the Bucs -- I was there for a double-header against the Padres where he took a no-hitter into the ninth (broken up by Fred Kendall, father of future Pirate Jason). Brett had an RBI single in that first game, and was inserted as a pinch hitter in the second game where he hit a two-run triple to tie the game. Bucs won on a walkoff homer by Richie Hebner.
Unfortunate coda to that story -- Brett and Randolph were part of a deal with the Yankees that sent them and Dock Ellis in exchange for Doc Medich. Both Brett and Ellis had better seasons the next year than Medich, and of course Randolph went on to an excellent 18 year career and over 2,200 hits.
At the time, trading Randolph looked like the smart move for the Pirates, as Rennie Stennett looked like a star until he broke his leg in 1977. He wasn't the same player after that.