It really depends on what you're using the unit for. If your saurus are just going to be an anvil, that sits there soaking up damage, a champion comes in handy against any big nasty heroes or lords. You can challenge them out, and for that first round of combat you know you are only losing one model, helping to preserve your unit's size. While you can almost buy another saurus warrior for the same points, by challenging out a horrible character you can easily preserve an additional 3-4 models in that first round. They also lose out on their Thunderstomps, if they have any, for that round, and so to me champs are always worth it on infantry blocks that you intend to preserve the size of.