I'm guessing Stallman is completely disgusted by the possibility of dual licensing. But the legal copyright holder has the right to establish whatever licenses they wish, whether Stallman approves or not.
As to your question, you've already agreed not to charge for your patch by the fact that you've utilized the GPL to access the code in the first place. That's one of the points of the GPL after all.
That's interesting, thanks for the citation. I was basing my opinion on his well-known hate of proprietary software, and his desire to make all software free. Dual licensing works counter to that, because it allows someone to use GPL software without making their own derivative software available too.
Well, it's not "GPL software", it's software that's also distributed under the GPL. That's being freely distributed at all is a win compared to most commercial software.
And in fact, it could be argued that it's better (for rms' goals) than MIT/BSD/etc, since the proprietary license will impose some restrictions on its usage by other proprietary vendors.
I'm going to disagree about Stallman. I suspect he would prefer all-GPL-everywhere, but I think he's pragmatic enough to think that dual-licensing (with GPL being one of them) is better than nothing, because it still means the software is out in the open, for anyone to download and use who wants to adhere to the GPL, so it's still a win for Free software. Some companies might choose to purchase a proprietary license so they can use it in closed-source software, but for others they're able to download and use it for free under the GPL.
Remember also, Stallman and co. did come up with the LGPL, which itself is a pragmatic compromise to allow GPLed libraries that can be used in non-GPL applications: under LGPL, you have to share any changes you make to the library itself, but the rest of your application can use a different and/or proprietary license.
As to your question, you've already agreed not to charge for your patch by the fact that you've utilized the GPL to access the code in the first place.
Nothing about the GPL stats that you can't _charge_ for your software, in fact RMS explicitly encourages people to make money from free software. You just can't restrict the buyer from redistributing it once you sell it to them.
True, charging not explicitly prohibited. But since everybody you distribute to is free to redistribute it again without charge under the terms of the license, it does put a practical limit in place.
As to your question, you've already agreed not to charge for your patch by the fact that you've utilized the GPL to access the code in the first place. That's one of the points of the GPL after all.