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I think "=" could harm a query param. "?{name}={value}" when name has an '=' will result in an ambiguity between where the name ends and the parameter begins if "=" isn't encoded. Not the worst problem I can imagine though...
Per http://lists.w3.org/Archives/Public/www-ws-desc/2007Mar/att-0083/20070329-ws-desc-minutes.html#item07 we will add "=" to the lists of characters that SHOULD be encoded in bullets 3 and 4 ofr 6.7.1.1.