Ok, I'll articulate:
Merger deals, particularly all-shares "merger-of-equal"s, have a significant failure rate, and famously spectacular implosions (Time-Warner-AOL, Chrysler-Mercedes) are famous for damaging some of the highest market cap companies in history. Many things can go wrong with a merger: Integration risks, loss of valuable individuals, lack of experience in each others' business, outright fraud, loss of customers, etc. Also, the buyer never really knows if the seller has artificially inflated metrics or engaged in unsustainable practices leading to the merger (long-time readers of this thread remember what happened with Salix). A commonly sited figure is that 70-90% of mergers fail. (See Harvard Business Review:
https://hbr.org/2011/03/the-big-idea-the-new-ma-playbook).
To offset this failure rate, for any credible merger, there has to be significant value creation: Because there is some substantial probability that the combination will be worth
less than the sum of the parts, there must be an offsetting probability that it will be worth
a lot more.
This has nothing to do with share price, or premium, or shareholder votes, or deal parameters, or if it's cash or stock, or debt, or any financing consideration; it is just the simple question of whether the two businesses can make more money operating separately or merged.
The proponents will focus on the merger's value creation. They don't go around saying that, historically, X% of mergers failed. The investors already know that bit. If the value creation they articulate is not convincingly high enough to offset the failure risk, the shareholders of at least one company would be better off keeping their company independent and will be disposed to vote against the merger.
In this instance, the value creation is very limited and entirely based on (possibly unsustainable) tax inversion savings. The premium is broadcasting that the shareholders are unconvinced, and expect enough of their peers to be voting against it.
(BTW, there has been some posting making a connection between Pfizer share buybacks and the merger premium. There is no connection. Yes, every dollar used in a buyback reduces the number of outstanding Pfizer shares and therefore increases the percentage Allergan shareholders will own in the eventual merged company; but it also reduces the value of that company by exactly the same dollar. So the effect is a perfect wash.)
(I'll crow a bit about Valeant after the market closes).
Dan.