My argument is directed to the rational investor who simply...

My argument is directed to the rational investor who simply wants to maximise his return on investment.
To follow on from your final sentence, both parties may well feel that they benefit from this particular transaction. But my argument is that, in this case, by buying above intrinsic value, the supposedly rational investor does not actually benefit from the deal as much as he thinks he does.
It's very difficult for the long term holder of Argo, who, out of habit, pays "x percent" above the intrinsic value of Argo stock through rights issues, SPPs and DRPs, to outdo the long term performance of the underlying holdings in Argo's portfolio.