On the face of it, Google would have us believe Panda and Penguin are two separate algorithmic updates, former seeking to demote low-quality sites and the latter demoting sites with questionable backlink profile.
If it is agreed that backlinks are the means to put forth quality content, not an end by itself, wouldn't it then follow that Penguin should be tagged to Panda?
The idea struck me (not sure if it is not already discussed in some other threads here), when I saw some sites with blatantly questionable backlink profile, yet giving a miss to Penguin. I had an opportunity to see the user metrics of few such websites, and it was apparent that user loved those sites (low bounce rate, high avg. time spent, number of pages per visit...)
Why not? If people love a website, shouldn't Penguin give way to Panda?