To paraphrase our DMs....if you're backing contenders in_running then a marginally better price is good, but if you get filled at 1000 it's probably on the deck. The proposal I heard about would allow a max %improvemnt to be defined.
And you might be ok backing £100 at 2.5 but you wouldn't necessarily want that £100 at 50 or 100+. Liability staking is calculated at your offer price not the matched price so this would allow you to submit variously staked backs with variously effective ranges to get around that, and the problem of having oversized or wasted back bets.