The average free agent gives you 1 additional win per $8-9 million. It's an expensive way to get better, but no, it's not strictly PR. If it was strictly PR it would have no added value whatsoever.
The Red Sox have had a ton of free agent busts, but they've also had plenty of free agent successes that have contributed directly to 4 titles. We can go over that as many times as you want.