æthereum is functionally identical to Ethereum, but rather than purchasing ether with bitcoins, you can freely claim your share of æther by signing an æthereum address with your bitcoin private keys.
That's basically what happens when Bitcoin forks, isn't it? After the fork there are two block-chains, and people who owned coins before the fork now own coins in both block-chains. The alt-coin is a more radical fork than the Bitcoin community would normally consider, but conceptually it's still a fork of Bitcoin.
Another useful property of this technique is that it makes the bitcoin blockchain robust to 51% attacks.
In the very unlikely event that the bitcoin network came under an unrelenting 51% attack, people would simply use the most promising clone. Since users have the same slice of that pie too (unless they traded), there is no need to "panic trade into litecoin," for instance.
That's mainly helpful if there isn't much growth in the user base between the fork and the attack. If æthereum launches, and, say, 2 years goes by, during which Bitcoin sees a 10-fold increase in users, and then the attack happens, 90% of Bitcoin users will be holding no addresses that they can turn into æthereum coins.
(Numbers picked for simplicity. I expect and hope to see exponential growth in the Bitcoin user base over the next several years. Even if it's 4x in 5 years rather than 10x in 2 years, the benefit of what you say here will fade.)