The above comments are valid but there is another possible reason to not rely on Time Machine alone if you are use certain software.
Any software that uses a monolithic database of some sort can be a problem. Microsoft's Entourage is probably the best example. It crams all of the mail and calendar information into a single file buried in the Microsoft User Data folder. The size of this database can grow to multiple gigabytes in size. If a single new message comes in between Time Machine backups, then entire database is seen as changed and Time Machine will try to copy the entire file again.
Additionally, since the mail database is always open by a background process, backing up a user database with Time Machine used to corrupt it. I don't know if this is still a problem. Probably so. I've always recommended excluding the Entourage database.
Other applications like VMWare and Parallels use large disk images to hold their client operating systems. These have the same problem. Time Machine may try to back up multi-gigabyte files every hour, quickly filling the backup drive and leaving the user with less protection than they might imagine.
There is a chance that Filemaker Pro files might be corrupted by a Time Machine Backup or that the Filemaker files might not be backed up as regularly as expected. Filemaker advises excluding them from Time Machine.
So if you're using any of these sorts of applications, you need to exclude these files from Time Machine and find alternate methods of backing them up.