There seems to be lacking a uniform consensus on this issue, and I'm not sure if this is due to an engineering, cultural, logical, or legal reason. Half the guys I ride with say to only use it once, then discard. The other half (myself included) re-use it constantly throughout the entire life of the chain.

What is the "official" recommendation on re-use of a KMC Missing Link?

Further, as it requires 25€ of tools to install/remove, what is the point of using a link only once if I can just use my existing 25€ tool to install/peen a new chain pin? In the hands of a capable mechanic, there is really minimal benefit I can see in it's use if it cannot be re-used. If it should be re-used, which it obviously CAN, albeit this is very different than SHOULD, then it's obviously of great benefit and makes total sense to invest in the tools to use it.

For the record, the UCI Continental team BigMat-Auber 93 re-uses their links.

I know many of you will respond by saying "only use it once" or "I re-use the link 17 times over the course of the life of the chain." But what I'm looking for is WHAT and WHY the official recommendation is what it is.

The concensus for KMC and Wipperman master links is that the link should last the life of the chain it's installed on but a new chain should have a new link. The "multiple-reuse" recommendation means it's OK to remove the link so the chain can be cleaned off the bike and the same link used to rejoin the chain after its clean.

SRAM does recommend not reusing their 10-speed master links even on the same chain once it's removed but there is no reason given. I've heard it's a patent issue, not a mechanical one. They make no such recommendation for their 8 and 9-speed links.

+1, Link pins wear the same way as all the chain's other 110 pins. Discounting any "official" policies by the makers, the link and chain should stay married for life, so their wear condition stays matched.

IMO, if a link cannot be reused safely, then the chain should be kept on the bike unbroken until replacement time.

You want "official" but ask here?! Weird.
What does KMC's website say, or their employees when you asked them?

Officially, some specific models are reusable and some aren't, according to KMC's web site. Unofficially, I've never heard of one failing, regardless of model, even when reused.

Once a link spreads, the links behind it cease to work and the stress increases on the links ahead of it, perpetuating their wear. If you put a worn link into a new chain, it will act as a seed of damage to the links in front. Proceed at your own risk.

I think the question is about re-using in the same chain after removal for e.g. cleaning, not re-using it on a new chain.

As a non-capable mechanic I think the main benefit is that it's really easy, reusable or not.

BTW, to the OP, I don't think there's a real need to buy a tool to install it (IMO). Put the link in the top run, then a little bit of pedal pressure with the rear brake on seems to work fine.

It's easy to install without a tool, like Earl said. Removal without the special pliers is also possible, although it takes a little more effort and/or technique.

This has been covered numerous times by KMC employee and forum member Papa Ado. Do a search. You don't need a consensus, just the proper source.