haha. I haven't heard the expression before. It is interesting I often don't like throwing out clothes unless they are worn out.
I suppose you could say exactly what you just said in English, although it's not a common expression in English. If they got dirty though, couldn't you just wash them? Do you mean if they get dirty without washing or if they get permanent stains which don't wash out?
The reason I am throwing them out is not because they got dirty. I am throwing them out because there is a big rip, which I do not intend to mend. I am just going to wear them (since right now they are still relatively clean) to the point when they get dirty. And then throw them out without having to bother washing them. Hope this explains it.
Oh, sorry! Maybe I misunderstood. So it is because they are worn out then, right? How about saying something such as "There's no need to mend them (or there's no point mending them). I'll be throwing them out soon anyway since they are an old pair."? (or does it have to include the fact that they are going to be dirty?)