i personally farm 8-10 level 10 NPC daily, food supply is secondary.
the best thing u get over here is hero exp.
here are the benefit
1) u build troops alot faster, by alot fast, how bout 1 sec 1 archer?
2) high atk hero make waving or defence breaking alot easier as well, just calculate it well
3) its average 1.8k prestige per level 10 NPC! not that prestige is important but it get's you a better alliance maybe?