Just to be very clear (because the term "merge" can be misleading)...
The coins are not merged into some super coin (i/e it's not a blockchain take over). The blockchains remain completely separate.
If ZET implemented AuxPOW against Uno, and a pool set it up, what would happen is that miners at that pool could earn ZET or UNO, as their hashes have the opportuniy to solve blocks for either coin, or both simultaneously, depending on whether the difficulty of the hash meets or exceeds that coins' difficulty.
If other coins also implemented AuxPow against Uno, then the miners could earn UNO, ZET, abc, def, and ghi coin( for their mining, as their hashes might solve blocks for other coins. It makes mining more profitable. The question becomes which coin will be the "parent" and which will be the "child." It does require a fork for the coin(s) implementing AuxPow, so clearly it's more convenient and less work to be the parent.
The thing that usually will kill the idea is the bad connotation a community peceives in being labled the "child" coin. As we saw with Doge when they merge mined with LTC, it will be seen by some as a kind of defeat, even when history shows that it is usually a prudent long term move.
Re BIG: I applaud the thinking. And Yes they have a very cool ticker. But I think it would prove to be a mistake for us to take on their problems Their block chain is stuck at high difficulty right now because they don't have anything like KGW (they only change difficulty every 30 blocks); there's a bad taste in everyone's mouth because of the way the dev abandoned them; they are not backed any more by bullion (if they ever really were); they're not on any exchanges that I am aware of; they have a huge premine relative to coins issued; is there any community left? I know their miners have all gone; it would take a lot of work by our devs to fix it (I know our guys won't do it, and I'd hate to even ask them).
Maybe instead of trying to build a mining ecosystem around Uno, we just implement AuxPow with BTC and be done with it. Uno's blockchain would remain separate, the specs wouldn't change; pools mining Btc can choose to mine Uno; existing pools don't have to setup merge mining (they could if they wanted); Individual miners could still mine Uno directly. This should probably be part of the discussion as well. The downside (as discussed above) is that everyone (pools, exchanges, you) would have to install a new client. And there's maybe some risk that Uno wouldn't get the "mind share" among larger pools that a large cap coin like NMC would get.
Here's how the hashrate for DVC (Devcoin, not Distributed Vault Coin) looks vs BTC (DVC is merged mine against Bitcoin).
https://bitinfocharts.com/comparison/hashrate-btc-dvc.html Even if Uno got 1/100th of the hash of DVC, it would be orders of magnitude above Uno's current hash rate.
Here you can see the effect of DOGE's hash rate when they chose to merge mine against LTC last September.
https://bitinfocharts.com/comparison/hashrate-ltc-doge.html Whichever way we go (or even if we do nothing), a decent market cap for Uno remains an important part of the plan.