Having <p> tags for the paragraphs in a multiparagraph comment would allow for more consistent styling via CSS than double <br/>'s.
Or maybe include in the tpl_comment template, the function which converts newlines to <br/>'s with an option there...
<p><?php echo $comment->comment ?></p>
...would become...
<?php echo JComments::nl2Para($comment->comment, NL2PARA_BR).?>
...where NL2PARA_BR is a const with other options available like NL2PARA_P, NL2PARA_DIV. This way you keep the simple BR functionality and allow template overriders to use P tags without an unnecessary additional str_replace.