Oh well, I've written so much in the comments I could wrap them up in an answer.
Basing on the definition of "approve" in the Webster dictionary: :
- Give sanction to; "I approve of his educational policies".[Wordnet]
- Judge to be right or commendable; think well of.[Wordnet]
- To show to be real or true; to prove.[Websters]
- To make proof of; to demonstrate; to prove or show practically.[Websters]
- To sanction officially; to ratify; to confirm; as, to approve the decision of a court-martial.[Websters]
- To regard as good; to commend; to be pleased with; to think well of; as, we approve the measured of the administration.[Websters]
- To make or show to be worthy of approbation or acceptance.[Websters]
(bold is mine)
The process of approvation implies that you make a judgment, the more pondered upon the better, on a subject and the you sanction that subject to be good enought for the standards and therefore can be taken as it is.
If you say you approve something, but at the same time that it's in need of improvements, you're quite contradiction yourself, as the process of approval - since there are no possibilities here of "approval but with some doubts", or however you say it in English - must mean you think that content is worth enough.
I know there are things that can be undone like you said in your comments, like votes (but only in a small timespan, and later on unless the post is edited), questions/answers and comments (within limits), but all those things have different natures. User are not compelled to give the best answer ever (nor the one they could), and problems may arise after comments or more infos, so it's ok to be able to modify it. Same goes for votes, but as you see those are closer to an "approval" as once the grace period is over, you can only change your vote (i.e. change your judgment) only if the answer/question changes, thus meaning you are responsible for the vote you gave, and can change it only if things behind it change (answer gets better/worse)
As for approvals, if you allow people to approve, but then change their mind and improve it, where is the need for 2 people's judgment then? why do I need another person's (competent) opinion if my approval has a lower value, and can be changed? we could skip the process altogether then, don't you think?
If you approve something you're required to think upon it, and think well, otherwise you're approving lightly and, effectively, approving something that might not be worth of approval. If you think upon it, you see if it's worth or not. If you don't think and start approving, you're not doing a perfect job there.
I see many times people approving without thinking too much, maybe where posts has only a single word changed while there surely was other things to change. This is, in my opinion, a "bad" behaviour, and giving the possibility to improve an approval only diminish the value of the process, as if it wasn't already underrated too often.
Sorry if I'm not entirely clear but English is not my first-language, hope I made my point though.