1) Convenience.
2) Conversion rate (from visitor to subscriber).
Not having an RSS icon makes it less convenient for the user and "out of sight, out of mind" guarantees you less subscribers as well.
Why less convenient? Well, consider your workflow:
1) Double click in the URL address bar.
2) Copy.
3) Type in or click on your RSS reader bookmark (e.g., Google Reader).
4) Log in if you haven't already.
5) Click "Add Feed".
6) Paste the URL.
7) Click OK.
I'm not saying that there is anything wrong with it. Just that I find an RSS icon less cumbersome.
If you are a technical blogger, not having an RSS icon means shooting yourself in the foot and providing a disservice to your readers (particularly now that some browsers have stopped autodiscovering feeds and presenting you with their icon in the bar).