As others observed this is a common condition in plants (where repeated hybridizations can result in even higher ploidy conditions, e.g. tetraploid wheat, sexaploid sedums).

But as you ask, why isn't it observed as often in animals, or indeed at all in mammals? One argument is that plants generally feature fewer tissue types, and their anatomy shows less interdependency of parts: they have a body plan of repeated units showing some redundancy. (Cutting off a branch is not like cutting off a leg.)

In other words, by many metrics of organismal complexity, plants are less complex than animals, such that their developmental programmes can "tolerate" relatively major disruptions to genomic organization.

There are some insights in this paper: http://www.ncbi.nlm.nih.gov/pubmed/15780745