I wouldn't consider kunai more powerful than higher level techniques, yet they have been shown to be able to kill if used correctly. I hold the same belief for explosive tags.
I am sure they would come in handy if conserving chakra is an issue, as well as setting traps (Shikamaru's trap when fighting Hidan, Konan's trap when fighting Tobi, etc).
As Zetsu said, "Someone with kunai can be beaten by someone with pebbles if they are thrown by a master". If strategy was used then they would be effective, but blindly throwing them at someone won't do much good (as they are easy to dodge).