I always suggest changing seals, water pump, tensioners ect... when doing a timing belt if replacing the timing belt on a car where the timing belt replacement is a very labor intensive operation. Say for example the timing belt change interval is "change belt every 100,000 miles". If you have 100,000 miles on the engine now you are expecting the oil seals and water pump to last until 200,000 miles. If the oil seal, idler bearings or water pump fail before that you must duplicate all the labor to replace a component that was accessible during the initial labor operation, and of course cars never fail or break down at a convenient time or place. Of course economics come into play and a persons finances may not be able to cover such a comprehensive preventative maintenance. Many repair shops will lure a customer in with a "low price" of just replacing the timing belt itself.