Is it necessary to backup iPad to iMac if you're already backing it up to iCloud?

My iphone and iPad are both backed up to iCloud, but is it still important to back them up to my iMac? I have an extensive iTunes library, but it is backed to two external hard drives. My whole computer is backed up to the two external drives. So, with that in mind, do I really need to back up my iOS devices to my iMac?