The main reason is responsive design uses CSS and it doesn't require in-depth template changes. This makes it easier to adapt to mods and custom edits. Since our user base likes (as well as us) customizing their forums, responsive design will not make it harder for them.
Well, I can't necessarily agree with that. It depends on what they want to do. If they only want to do very basic customisation then sure, it probably wont make it harder.
The more customisation they want to do, the harder it will get, simply because responsive means more CSS and more complex CSS that requires a lot more testing to get it to work consistently (as I'm sure you're already finding out

).
However, if people want something that will work on all devices, they're pretty much stuck with dealing with more complex coding. You don't get something for nothing.
Even mobile themes are more flexible but they brings support / modification issues with them. That was the reason why we chose responsive design over mobile theme.
I have mixed feelings about it all. The problem with only using responsive is that it basically leads to a certain amount of bloat. Usually there is heavy reliance on using display: none; to hide a lot of stuff on small screens. So you have to include the extra code for that in the CSS, and the device still has to process all the extra CSS, and all the hidden HTML and javascript, even if it doesn't use it.
In some ways, pure CSS responsive is the least efficient way of dealing with the problem. A lot of people probably don't realise this, because they just see the latest buzz word and think it must be awesome. Like anything else, it's a trade-off. Rather than thinking of responsive as the best way of handlng things, it's probably more accurate to think of it as the least bad way.
If there was simple and reliable detection available, not via media queries (which are limited in application) but by another method, then it would make sense to use this to drop unwanted content via PHP conditionals. This would reduce the amount of CSS required, and reduce the amount of unwanted HTML and javascript that mobiles had to process.
The problem is that due to the piecemeal way in which every manufacturer does their own thing, there is no simple and reliable way of automatically detecting all devices and using that to apply PHP conditionals. IOW, we're still stuck with the age-old problem of coding for the web: lack of consistent and useful standards support. Hey ho.
