I get ants in the greenhouse all the time. Almost always they're nectar-feeding ants.
IMO the benefits:
--Free food... ants feed the plants
--Keeps leaf diseases down. The extrafloral nectaries are a source of sugary food for black mold and mildew. By eating and removing the sugar, the ants keep the leaves clean and I don't get ugly stuff marring the apprearance of the plants.
--Really, really, really cool to show to people. It is easy to say "N. bical lives in symbiosis with ants!" The minute people see it actually occurring, however, they are awestruck. I have seen this time and again.
--Many nectar-feeding ants also farm pests like aphids, mealybugs, and scale, which make sugary excretions. They will truck nymphs of these pests into the greenhouse through small cracks and deposit them on the plants. Minor annoyance for me, compared to the sense of wonder you experience at seeing the mutualism right before your eyes.