[Thremp]

We can use imaginary Derek Jeter.

.318 when he rakes we walk off...
.071 when he swings we load the bases...
.02 he grounds into a DP

We can assume best case 100% bunt success. And say that half the time there is an out runners advance.

.318+.071(.894)+.305(.426)+.305(.695)+.02(.263) = 72.8%

So if Jeter were to be 100% bunter, it would still like be +EV for him to swing unless the person fielding the bunt was McKlutz and would have an error >3%

If you wanna jazz this up, find a better bunt rate and a how often a runner advances to 3rd.