Every additional backup adds another layer of security against things going wrong. Every encryption adds another vector for things to go wrong. From a probability point of view, two encrypted backups are very likely safer than one unencrypted backup but one unencrypted backup is safer than one encrypted one (from a things-going-wrong perspective not from a keeping-your-data-private perspective, there obviously encryption is a boon).
I for once would add at least one more backup the moment I start encrypting things (both main drive and backup drive). And will MacDrive support encrypted backups? Being in a situation where all your backups are on HFS+ is pretty much standard, and being in a situation where you 'only' have a Windows PC to access your backups is a possible scenario for quite a number of people.