Very true. All too often a client’s brief is seen as sacred and suggesting that actually there might be a better way of doing something can be a very tough thing to do, especially for more junior designers. Ironically in the long term this can actually help to build better client relationships because the client can see that you’re doing what’s best for them, rather than blindly following a brief. It’s also worth noting that this situation doesn't just arise within agencies, it’s equally applicable to internal clients as well.
There was an excellent keynote presentation at UX Cambridge covering this called “Working beyond the brief”. I can’t find the slides but Francis Rowland has put together some sketchnotes (https://www.flickr.com/photos/francisrowland/21448853765) that are well worth a read.