I'm pretty sure I read somewhere recently that Google are getting wise to this (or more to the point already knew and either didn't know how to stop it or chose to ignore) and at some point will stop it by dropping everything off a domain when they see change of ownership. If true it makes sense but obviously penalizes people
legitimately buying existing online businesses.
It may never happen, or it might be years away, but anyone considering buying lots of domains just for
SEO juice reasons should at least recognize that risk.