If the bot is right, then it must be the long-term value of the spare on the sixpoint (good for playing potentially awkward fours and fives). The 6/3 play is less flexible in comparison. Kit Woolsey is famous for his statement "you can't say enough good things about a spare on your sixpoint".
The four homeboard blots after 4/1 are not really a concern here, as they figure to get covered soon, before any hitting might start. Flexibility might be very important after playing 13/10(3), as clearing the mid, the ten and the eight safely is no walk in the park.