the cache is a temporary holding place,and storage for things that are loaded frequently. basically,not restoring the cache would mean websites that you load frequently,and even things you load from the phone,would be slow the first time after restoring the backup,until the cache is built backup.
not backing up cache wont really hurt anything at all. the important things are boot,system,and data,and if you move alot of apps to sd then possibly .android secure(i personally dont use this feature,so i dont back it up or restore it)