Consider the Malicious Software Corporation (MSC). Consider work X by
author Joe. MSC holds patent A covering X and patent B covering something
else. Valiant Defender (VD) holds patent C covering X.
Normally, MSC can sue any user of X for infringing patent A. With the
narrow patent-lawsuit-termination clause, doing so will cause MSC to lose
its copyright license for X. If MSC doesn't copy/distribute/modify X, it
won't care. If MSC does, then this might hurt it a bit.
Suppose MSC sues VD. VD could try to "fight back" with patent C; this would
only work if MSC uses X, of course. With the narrow
patent-lawsuit-termination clause, this "fighting back" would cause VD to
lose its copyright license for X as well.
I suppose some people might consider that to be bad for free software. (I
don't.)
Now consider the case where MSC buys Joe's copyright. What changes?
With the narrow patent-lawsuit-termination clause, MSC will retain its
ability to copy/distribute/modify X. This will not change its ability to
sue anyone over patents A or B. This will not change VD's ability to
"fight back" with patent C (or not), which depends on whether MSC uses X.
This will not change whether VD's copyright license for X is terminated or
not.
(Essentially, by buying the copyright, they would have gotten themselves a
special license to avoid the patent-termination clause, and that's all.)