Placentas Love Horizontal Gene Transfer

From a report titled "Mammals Made By Viruses":

What made syncytin peculiar was that it was not a human gene. It bore all the hallmarks of a gene from a virus.
Viruses have insinuated themselves into the genome of our ancestors for hundreds of millions of years. They typically have gotten there by infecting eggs or sperm, inserting their own DNA into ours. There are 100,000 known fragments of viruses in the human genome,  making up over 8% of our DNA. Most of this virus DNA has been hit by so many mutations that it’s nothing but baggage our species carries along from one generation to the next. Yet there are some viral genes that still make proteins in our bodies. Syncytin appeared to be a hugely important one to our own biology. Originally, syncytin allowed viruses to fuse host cells together so they could spread from one cell to another. Now the protein allowed babies to fuse to their mothers.

It could be that syncytin actually appears in viruses and is utilized for this purpose.  The article is really vague on this point, which would be interesting to know one way or the other.  The article seems to be saying that 8% of the human genome bears "all the hallmarks" of viral genes and are presumed on this basis to be viral in origin. Do the "so many mutations" refer to the differences between the animals that share the viral "junk" or between the viral "junk" and known ancestor genes in known viruses.  Toward the end:

The big picture that’s now emerging is quite amazing. Viruses have rained down on mammals, and on at least six occasions, they’ve gotten snagged in their hosts and started carrying out the same function: building placentas. 

Lightning has struck at least 6 times.  Curious and curiouser.  You'd think selection would work against tampering with essential reproductive tissues.  Well, think again.  Why is a breakdown of cells into multi-nucleate tissue is important for some animals and not for others?  There should be as much interest in what purpose is served by the difference than speculating on the mechanism for the difference.  The article says there are "all sorts of explanations for why that may be"... Why is it so hard to figure out why natural selection favored it?  What heuristic provides the predictive power for where the true explanation lies?