Having prayer makes you much weaker for your combat level than having it... (only against people your own level)
Not having prayer means
You can pk with range to melle without prayer switches
you can PK in high risk worlds and have a big advantage on other players
you can have much higher stats for you cb level.
If you stay 1 prayer you lose 15% of your offensive.
but you gain a much stronger defense (even on pures) and much more efficient pk stance
Explained
Example of a simple build (level 49 combat)
Which would you rather... way high HP and less chance of getting dropped...
or prayer