I think you'll find that even though you know a lot of channels and people that use FISH, they are still a very small minority compared to those who don't and who don't care about encryption. Of course, that doesn't necessarily mean that mIRC shouldn't consider supporting it. However, I can quickly see issues with people who have no idea what FISH is who suddenly can't understand each other if it was built into mIRC.
You don't see that issue now because people who are using FISH have actively gone out and went to the trouble of getting it set up, so they know what they are doing with it. The typical user could quickly become confused as to what's going on if it's built in.
Also, if you check the Developers Forum, you'll find a new method of using FISH that patches mIRC in memory rather than the file itself. That allows FISH to work on new versions of mIRC without waiting for FISH to receive an update. From those who I know who use it, I've been told it works well. It does have a few issues that are being ironed out still, but it still seems to work well. You might want to give it a try. I don't use it, so I can't give you specifics on it. Anyhow, if it works as well as it seems to and is as easy to use and it sounds and it doesn't require updating every time mIRC updates, it may be easy enough to use and less of a hassle and may be just fine as-is without needing to build it into mIRC. Just a thought.