The real problem isn't backing up only certain tables. It's restoring only certain tables. While you can put them into a filegroup on their own and back that up, restoration can be a bear. If you really only, ever, need certain tables backed up, you might be better off looking at simply exporting them to a file. In general though, you'd be much better off getting full backups because recovery is so much easier. What is the business requirement you're attempting to meet?