I'm pretty sure this isn't going to be coming back. They already tried to ax it with 5.0 on the 4S but brought it back with 5.1 because people complained.
BUT there is a technical reason behind it: with 4G LTE becoming more prevalent now and soon to be ubiquitous, the carriers (at least those in the U.S.) have openly announced they are going to be shutting down their EDGE network in the near future to repurpose the spectrum/frequencies it runs on to be used for LTE/3G to accommodate for the greater demand of data services. This could very well happen before iOS 7. If they don't remove it now, they'd have to with 6.1 or something. If you don't have an EDGE network to connect to, turning off 3G would be you would have NO connection whatsoever.
When EDGE gets turned off, you shouldn't have any need to flip the toggle as the idea is that the EDGE network you would have been trying to connect to would then be 3G/LTE anyway. Again, that's just the idea, though. We'll have to see if it works that way in practice.
But get used to it. The 3G toggle likely ain't coming back. Now the LTE iPhone may have an LTE toggle to replace it, though...