#24913 (comment_text filter used differently in two places in core) – WordPress Trac

In the first (/wp-includes/comment.php), the $comment variable contains just the comment text. In the second (/wp-includes/comment-template.php), the $comment variable contains the actual comment object itself (based on the return of get_comment( $comment_ID )).
So the filters are filtering the same fields. The issue then becomes the number of parameters and that, too, is not a bug.
The first comment text filter is used specifically to filter the text of the comment before things like inserting it into the database. The second is used when filtering the text of the comment before displaying it on a page – this follows the pattern of not trusting data before inserting it in the DB and also not trusting the data when pulling it back out. Using the same filter here helps us use exactly the same rules to filter the content both pre and post DB.
If the problem is the number of fields, simply make the second parameter of your function optional:

function filter_text( $comment_text, $comment = null ) {
    // Do something to the comment, possibly switching based on the presence of $comment
}
add_filter( 'comment_text', 'filter_text', 10, 2 );

If you only want to do things when the comment is inserted into the DB, just verify that null === $comment in the function above. If you only want to do things when comments are displayed, verify that null !== $comment.
IMO this isn’t a bug but a pretty powerful differentiating factor between a filter that can be used in different places to do either the same or different things.