let's draw up some sample scenarios for what i'm suggesting:
let's say "pvp champ" runs across "noobie mcrabbitfur" out and about, and starts a land fight. let's assume here that they both have equal amounts of equal quality water on hand (though they don't). pvp champ has 2x the UA of noobie, and starts the fight.
pvp champ punches noobie for like 70% of his def bar. noobie freaks out and takes off running and drinking water. this is his only window of opportunity to escape, as once champ lands that second punch which knocks through his def and gives him damage (no matter how small), he's done running. walk speed only, now. pvp champ will probably take him down anyway, but noobie still has a reasonable window to get away. pvp champ will need a few jumps, or a NPNG, or thunders + bee sting, or something, but he'll figure out a way to get it done.
now, let's take the case where noobie got extremely lucky with his bluebells and flotsams and has 2x the UA of pvp champ. champ will go in, but punches will only take off what like 15 or 20% of def bar, or something like that? this means that if noobie still wants to run, champ will have to get off that many more punches and offense moves before noobie is done with run speed. thus, an incentive to higher UA in this situation.
it's left as an exercise to the reader to extrapolate into 2v1 situations, and so on, but off the top of my head i don't see any particular problems coming up with it set up this way. feel free to offer any bad counterexamples or exceptions that you may be able to think up.