You're not completely wrong. You're not going to get more SPL out of a sealed box. SPL is more a measure of pressure than sound, thats why they use ported boxes to create as much pressure as possible.
Now more SQ out of a ported box ... maybe. Depends on the box if you can get a box to have a flat bl then it will have pretty good sound quality. But for most people a sealed enclosure will sound cleaner, especially in situations where the box is in the cab of something.
Also one of the big things in SQ is tuning, people just don't know how to or just want something to make noise. In that sense most woofers can become a SQ sub even though their design isn't for it you can still get dececnt SQ out of them in the right box etc.