It's a very good idea to have an up to date bootable clone of your startup drive. There are 3 main reasons.
1. Your startup drive may fail. All drives will eventually fail, it's just a matter of when.
2. Your startup drive may get corrupted. This can be caused by badly written software, system crashes, etc. Although viruses are rare in OSX at this time, who knows what the future may bring.
3. You may do an OS and/or software update or install that compromises the performance of your computer or applications.
I use Carbon Copy Cloner
It's free and does a good job.
Update, it's no longer free.
Superduper still is
I recommend testing your clone after you create it.
I clone to a second internal drive on my mac pro which allows me to get back to work in a matter of minutes if things go haywire.
For a drive to be bootable on an intel machine, it needs to be formatted with a GUID Partition Table.