Question: What the advantage in buying non private proxies list ?

So, like the title says. i see alot of ppl offering here proxies that they scan. there are alot of public websites that are being updated with such proxies. so what the advantage in buying those ?

I'm asking that because atm i got my own script checking those free proxies websites every hour then checking the proxies themself for speed, anonymous level and etc... i can generate a list of around 100-150 proxies that are "ok" for my needs but i can use a better and faster ones (they are being used for xrumer).

If you have some kind of service to offer you can post it in here/pm me as well.