Agreed. You can't generally won't get the same plant from a commercially produced tomato. Instead you will get all kinds of random stuff. Some of it may produce good tomatoes, some may not.

If you are going to go to the effort of growing a garden it's probably worth spending a buck or two and getting some Heirloom or open pollinated seeds. These will breed true and you can replant the seeds from the fruits and get the same plant as the parent.