Consider the following hypothetical:

Changeset 1:  A real change associated with this issue that needs to be reviewed.

Changeset 2: A mistaken change that should have been associated with a different issue.

Changeset 3: A second mistaken change that should have been associated with a different issue.

Changeset 4: A third mistaken change that should have been associated with a different issue.

Changeset 5: A CPKRemove for changesets 2, 3, and 4.

Changeset 6: A second real change associate with this that needs to be reviewed.

Changeset 7: A third real change associate with this that needs to be reviewed.

Using the existing 'begin/end' radio buttons, a reviewer has no way to review the authentic changes (1, 6, 7) without also including the files that were mistakenly associated with this issue (2, 3, 4).  Use of check-boxes to 'include' transactions in a code review (either at creation of the code review, or during the Review process) could remove these irrelevant files and transactions from a review, allowing users to focus on the actual code changes in question.