Agile could definitely still offer value, although it might not be worth switching if what you are already doing is working well.
Agile offers a lot of value in situations where priorities are unclear or change frequently, but I find it has other benefits as well. I find that Agile methods tend to improve developer morale (when used correctly), reduce unnecessary process, and increase communication both within the development team and between the development team and the business owners. The increased communication, IMO, is the biggest advantage to Scrum / Agile, as this leads to better risk management and more efficient processes.
I think the greatest risk is that Scrum takes a few sprints to get working smoothly. Scrum relies heavily on the ability of developers to make good estimates, and this is a learned ability. Also, most teams need to adjust the rules a little bit before it works well for them. Scrum provides a great process for this through the retrospectives, but the first two to five sprints can be a little discouraging. After that, I've generally seen teams pick up significantly and greatly increase their efficiency.