You still want to cut your exposure. If you were resting a bid for 100 shares at 99 for 100 shares and an offer for 100 shares at 100, and the market's falling, then sure your highest priority is cancelling your bid at 99 and putting in your new one at 98, as well as putting in your new offer at 99. But you also want to cancel your old offer sharpish, otherwise if the price shoots up to 101 then you'll end up selling 200 shares instead of 100 and exposing yourself to more risk than you intended, potentially exceeding your position limits etc.

Also most of the cost is fixed. If you've already built a super-fast cancelling system that can cancel an order in 8 microseconds, there's no value in running a slower cancelling system in parallel for your less important trades, you'd just run all your cancels through the same system.