Maybe next year's refresh will come with some low end quad, but I wouldn't bet on it. As for the CPU speed: since you are someone who writes code, I would have expected that you already value even the tiniest speed increase, since it can lower compilation times ( sure, many would say: "what do a few seconds matter?" ... well, adding those seconds up, you'll conclude that IT MATTERS A LOT on the long run ).
That being said: If money is a problem, then don't get the upgrade. It will surely not impact gaming in a serious way... your integrated GPU will be the bottleneck, not the CPU. VMs could benefit from a slightly faster CPU, but don't expect too much... having a crapload of RAM would only allow you to open up multiple VMs, but it will surely not guarantee that you can use them all simultaneously in a decent manner ( CPU being a bottleneck eventually ).
I'm a software engineer myself and my 13" rMBP is my main machine. I do mainly Flex / Air and Java development, but from time to time, I do need to write some native extensions in C / C++ / Objective-C ( + the random mumbling around with new tools / frameworks / languages ). As for games, I have really only tried Dota 2 under bootcamp and it runs very well on almost maxed out settings at 1680x1050 ( for sure I can push it up more, but there is no need to increase heat output just to see a few leaves in more detail or bugs / butterflies, whatever ). I might be giving StarCraft 2, Diablo 3 and WoW a shot too in a future ( the HD 4000 can of course run all 3 games on Low settings decently - at least ).
PS: 13" rMBP runs significantly cooler than my late 15" cMBP. Even though it gets hot while gaming, I find it a lot more bearable than the 15" cMBP. Btw, I do use the notebook 99% of the time on my lap...