Totally agree. A good, balanced diet should rule out the need for supplements. If there are any signs of deficiencies, then a change of food would be the first port of call rather than topping up with supplements. Having said all that, there are a few cases when I do recommend them. The supplements that tend to benefit the most dogs include:
Omega oils are essential for all sorts of bodily functions and yet a lot of foods contain fairly limited amounts. Some foods have some added but since they are added before processing and since heat can destroy omega oils, there's no guarantee that they are still there in the finished product. Good sources include fish oils, whole linseed or linseed oil, evening primrose oil etc.
For dogs with joint problems, devils claw can work wonders and has been shown in a few studies to be just as effective for arthritis sufferers as many prescription medicines. It is, however, quite pricey and so is almost never found in complete foods. Green lipped mussel extract can also really help.
Raw garlic (a small amount each day) is a great natural way of deterring ticks and fleas.
There are also a number of other supplements that I might suggest for more specific conditions or circumstances but most dog owners will never need to worry about them.
Every supplement seems to have as many advocates as critics and what works for one dog might do nothing for another. If you're interested in any particular supplement, make sure you get the right dose and then give it a try for a month or two. If you can see some positive effects then great, keep going. If not, drop it. Either way, make sure you post your findings on this forum!