Pretty much never worth it. There are a few circumstances where I will do it.
When it only costs 10 cents to get off 14 either way in NCAAF.
Last leg of a 3+ team parlay, might buy 1.5 or 2 to make it easier to hedge if I want to.
I think 5 is an underrated key number in basketball, I will buy onto or off of 5 sometimes for 10c.