Yeah right...
Just like in the old days, when there was no giving a weapon a bonus beyond +5...
Then there appeared a "Unearthed Arcana" which listed +6 Defender, amongst other things.
This false "limit" of +10 equivalence is one of the things going west in ELH.
The pattern is there, it is easy to see, and extrapolate. (+11 "bonus" weapon would "cost" 242000gp)
It is just a GOOD IDEA to keep a lid on things. I hope it stays at +5 enhancement. The other stuff can just get piled on, as far as I'm concerned.
Remember, it is up to the DM to give the players the 200+ days off from adventuring to make the +10 equivalent weapon...or leave it lying around the dragons hoard.