Pierce is highly underrated. I don't see the boss singing along anyone else (even though he/she tried to with Kinzie). And clearly not many people actually listen to what is said in the game, where the boss states that even though they pick on Pierce, he's a good friend. The boss appreciates him in his/her own way. In SR2, Pierce was a bit of an idiot, but in SR:TT and IV he really turned around.
This! Honestly, before I checked out this thread I did not realize so many people disliked him. I am a huge Pierce fan. His back-and-forth dialogue with the Boss is among my favorite. I also really enjoy when they sing together. There will be times where I'm doing something random like washing the dishes and I'll just start singing his rendition of "Bounce Like My Checks" from the SR:TT DLC, haha.