Let's take a look at the possible cons/dangers of cloning extinct animals:
1. It disrupts the evolutionary process
2. It brings animals into the world that may no longer have an ecosystem
3. It could carry a deadly disease we have no knowledge about. (This one is kind of farfetched, but its still a possibility)
Now let's look at the pros:
1. It allows us to learn more about out past
2. We could possibly get these animals to reproduce more of their kind or mate with another endangered species.
3. If there was ever a food crisis on the planet, this technology would need to be practiced and perfected for our survival. (Another longshot, but worth noting.)
For con number 1, I don't think the evolutionary process is so fragile that one of the species within it could destroy it. Our progess as humans is a part of evolution, and by association everything that we do is as well. Therefore, evolution would not prohibit humans from using their technology in this way, but demand it.
For con number 2, I don't think scientists would just set a previously extinct animal out into the wild to die without its ecosystem. They could keep it in some kind of artificial habit to study or mate with another animal. As long as they keep the animal in a decent environment, I don't see a problem with it.
For con number 3, we should approach new animals with caution. We should undergo any tests necessary to ensure the animals are not diseased or dangerous to humans. This should be fairly simple to accomplish, and is probably not a serious issue.
If I have missed any points please let me know.