Jon The PL won't allow the 3 promoted sides to be the following season's relegated teams for long. It weakens the brand.
The league is sold to a global TV audience as being exciting and competitive. The no relegation rule seems to work ok in US sports, and its basically applies to Rugby Union in this country. Investors like certainty.
Say that Leeds, Boro and us get promoted; i'd say that us & Boro would be straight back down without much of a fight. And if Southampton, Ipswich and Leicester/Wolves/Palace go down this season, I can't see a situation where at least 2 of those sides don't go straight back up.