I hear this a lot and after waffling on the issue over the years, I decided this year that I disagree... at least, as far as June as concerned.
This year, and again I stress this was 2nd half of June, I did not find the beach overcrowded. I found the water to be clean and the sand as well. The beach is long, it's sandy, no rocks and pebbles. There are plenty of beach chairs available and very easy to grab a beer or snack when the desire hits (or get served on the beach). And you've got places like Tahiti/Bora Bora with some nice tunes playing. It's too commercial? So what! I didn't come to Ibiza to get away from civilization. And if you're staying in PdB, it's damn convenient!
Now, we've made a point of visiting many other beaches all around the island, especially on our past 3 visits. I've got several of them I truly love. Some are good for a particular bar or restaurant, others have sandy beaches and shallower, calmer water, and some are nice quiet places to sit with a beer under an umbrella and stare out at the water and think.
But I don't think there's anything wrong with PdB!