Why do you need it? That's just [s]childish[/s].
I disagree. It is useful for editing posts. If you go back and delete stuff from a post that has already been replied to you can make things very confusing for future readers of the thread. By using strike through you acknowledge that something is incorrect/not what you meant to write. Far from being childish, this is the adult thing to do, as you allow your acknowledged mistake to serve as an example for others (some of whom might have actually shared your misconception).
Your point is well taken since you make it in that context. When visiting other sites, I notice it can be used childishly. Most of us here are mature enough to use it sensibly however. Although a bit of lighthearted clean humour does not go astray on an otherwise technical forum(s) such as this one.
In the absence of a strikethrough feature, perhaps users can adopt a workaround for the time being. For example, decreasing the size of the text and greying it out as qviri suggested (although this could still be considered ambiguous as different people may use the same colours to mean different things).
That is perhaps the worst example of a duck fried in oil that I have ever seen and tasted in my entire life. You the self-confessed chef of the century should be rightfully ashamed of yourself! How do you feel now, seeing as you served me this--this--thing?!
Perhaps that is more work than most users would want to do, so a strikethrough feature may be worthwhile after all.