Indeed - I am somewhat surprised by James's firm opposition.
I tend to agree with James on this one in that if the edit summaries are to be modified then they need a revision history.
Typos in edit summary are fixed by releasing an errata corrige in a subsequent dummy edit.
I question whether or not the ability to change edit summaries is really a needed feature though. I would prefer the approach that Nemo recommend of making a dummy edit.
For me it's less about vandalism et al. and more about the principle of revision tracking and audit trails. When you make an edit that revision is fixed and should not be able to be modified. This is one of the core principles that makes wikis work.
Thank you, Derric Atzrott