Fillers are there to fill in newer audiences that are new to the series, which is pointless when you buy a series on DVD.
Most fillers are poor, or are just a waste of time really... I don't mind a filler for a fraction of an episode, that is, more like a recap of the previous episode and not the whole entire events.
Some fillers are great, like in Ghost in the Shell Stand Alone Complex with the Tachikomas, it builds around their existence that gives more meaning to the over show. So it's really mixed...
Bleach and Naruto tend to be about the same for their filler episodes..