ah but all lenses have a use - you just have to do a bit more searching to find out what it is. I would also say that the 70-300mm tend to not be useful in the areas that they tend to get advertised in being used for -
For example the sigma 70-300mm is useless after abou 5-6m for getting reliable sharp results even when on a tripod. Within that distance a tripod mount and you get get some very nice sharp results, but the 300m end can still be soft on occasion. But take the lens and use it (on tripod) for flower heads and its a wonder.
You just have to find what the canon 70-300mm can do for you.
Also correct me, but IS and a tripod?? I thought that IS and a tripod is a complete no no as the IS starts to compensate for camera shake when it is not there and gives blurry results.