Why would an intelligent designer design these?

Some of the Burgess Shale animals were originally misclassified - and we can't entirely rely upon books like Gould's Wonderful Life that were written before that came to light.Hallucigenia for instance has now been identified as a member of a phylum that still exists - the "velvet worms" (and yes, the spines are on the back).http://www.ucmp.berkeley.edu/onychoph/onychophorafr.html

Yes - the whole "oh bugger, another new phylum" bit turned out to be wrong. Most (all ?) of the Burgess Shale stuff - although weird - turned out to fit into known phyla. Simon Conway Morris goes into this in The Crucible of Creation.