If this )linked to page) is a middle-routing page where most low level pages are cut-off the higher level pages (e.g. - if -- delete page - can not move throughout the site via other routes) then your link structure (or link hierarchy) is broken.
The benefit - branding - "brands" people remember, and the keyword domain can produce a litte intial boost to help in ranking.
The work-around - zero ill affect to start - therefore monitoring logfiles must be a religous activity. At some point in the future host the second domain and develop a specific site around the primary pages being linked to (off-site) of the one domain that has the more links.
Also - note: many large corporations started this same way.... Macromedia - Shockwave, Adobe - Acrobat, Apple - Quicktime. All of these secondary sites -- were part of the original site and subsequently move to their own location (really don't believe this was the logic reasoning behind the move - but the principles are the same).
The client I am referring to is an organization with a long name. They want several domain names registered that use abbreviations of that name. So while I can appreciate what you are saying about developing a site around the secondary domain names, I don't think it's really applicable in this case. All of the names need to point to the same place.