I think the message is clearly that you should only cross link if the sites would be of interest to the members of your site. For example, I might cross link my stamp collecting forum with my coin collecting forum because people who collect one often collect the other too.
On the other hand, if I owned a portfolio of 30 forums on various topics like cars, guns, foot fungus, basketball, minecraft, etc... then interlinking them would not make much sense.
One thing this makes me wonder about is how much all those branding links we get stuffed into our footers by the software makers, plugin authors and so on are hurting us with our Google rankings. I wish the branding-free options were more reasonably priced since they probably don't help the devs and might hurt us who use the products.