When you open the edit tab, as long as you don't press on the Save edit button, nothing will happen. So take what you need (without forgetting to cite your source ;) and just close the tab if you really fear to push the wrong button (Save edit instead of cancel).
Before 2000 rep: if you edit the post of someone else it will anyway have to pass by the review queue about suggested edits before getting changed. So if you made a mistake, a reviewer will probably see it and reject your edit (so no worries).
After 2000 rep: if you edit a post then the edit will be directly effective. However, you can always rollback to the previous edit. So no problem.
Conclusion: Such a feature seems not to be needed for the reasons you quoted. (maybe other but I don't know them).