While it's true that Harbaugh will adjust the offense to his players skillsets (see: early season 2015, and all of 2017), he is recruiting for his offense. He is evolving it into an NFL passing spread - hence Pep Hamilton and the bounty of wide outs and Gronk-like TEs that run like the wind. He will adjust some things to suit the QB, but mostly he wants a QB that can run an NFL offense. He's got two right now, and wants a third as much for depth as for a possible upgrade.
He didn't have the parts on the field in 2015, was subpar in a few of areas in 2016 (QB, WR, RB - as good as they were), and had to start over in 2017. In 2018 he'll finally have all the pieces, but they'll need some seasoning. He's already proven an ability to out-scheme Meyer's staff all over the field, and now he'll finally have the athletes to match at each position.
Patterson, if he comes and is eligible, may not start - Peters and McCaffrey will be valid competitors. But he will add a jolt of 'oh, shit - competition' that may not have been there otherwise...