Yes, fly sparging increases efficiency only slightly (usually), but those who WANT to do it for the sake of trying it will need a manifold in order to do so.
Doing a double decoction may not drastically improve a beer (there have been side-by-side experiments that have shown that even for several German lagers, decoction brewing did not improve the beer quality), but people still like to do it sometimes, just for the method alone.