I think that as you're not using them yourself, you wouldn't see it on your flist even if people were -- the ads are hidden from you. For example, I've just now experimentally turned ads on here in my LJ, but I'm expecting that you don't see them -- but people on free accounts, or on paid accounts who also have them turned on, will see them.

But yes, it does seem a bit unlikely. I guess people might want to show ads if they use their LJ as a public blog rather than in a personal community-of-friends type way. But are there any such people? -- presumably they wouldn't be on LJ in the first place, they would already have proper commercial blogs which have been showing ads from day 1.