I understand the definitional difference, I'm just not entirely convinced the distinction between the two is incredibly useful.
If you don't cross the threshold, you don't have an impact, regardless of how many things are pushing/pulling. The only time it's not useful is if you have a linear disad.
Suppose you found the following two cards:
"By recognizing the the Hu corollary to the Monroe Doctrine and withdrawing from intervention in Russian and ASEAN issues, Obama can signal respect for China's growing influence--a respect many in the CCP think is sorely missing on the other side of the Pacific. Such a signal of mutual respect can mend the current rift in Sino-American relations."
"Obama's recent declarations in India have pushed relations between China and the US to a breaking point. The world is watching, and Hu and the rest of the CCP know it. If Obama fails to restore good ties before his return home to the United States, it will be too late for reconciliation."
The first card says what can mend the relations. This is a link card. The second card says why action must be taken now. The second card is neither a link card (it says nothing about what will restore relations) nor an internal link card (it doesn't 'lead' to any further impact) nor an impact card (it just says 'why now').
Without the brink card, the disad becomes something that can be done after plan. So say plan is Chechnya and you read the link card without the brink. I just say, "Plan solves for people dying immediately, there's no timeframe on the disad. We can intervene now and apologize later and do something else to restore relations, so we win." With the brink card, you cut out the "and apologize later" component, because that's no longer an option. You have a reason why now is the key time in Sino-American relations, so their action triggers the disad.
If that's not useful, I don't know what is.