I was out bugwatching tonight and saw a spider (apparently Trachelas due to the body coloration and only front legs being red) on a mint bloom, sticking its head into the flowers like a ridiculous caricature of a bee. Many research pdfs state that a wide variety of spiders, including the Trachelidae, feed on nectar and other plant products. This was no surprise, as I have seen carabids, assassins, and even mantids gleefully eating sugary plant matter.
However, it is very likely that herbivory by predators has been severely underestimated and could play an important role. The assassin bug mentioned never caught an insect while being observed in situ, but frequently snacked on sweet plant exudates.
You may be onto something. I know for a fact the reverse is true: many habitually herbivorous mirids and lygaeids are opportunistic predator-scavengers on other insects. Orthopteroid insects are almost all omnivores. Thank you for sharing your observation.