Well that is a question I asked myself a lot during past 1 year. I see so many blogs using private ads and with most of ad spots filled with something or other. But I could never get my ad spot filled with something useful. Still I kept the ad spots. But then I realized that what I was missing by keeping those unpaid ad spots all along.
In between I tried several methods to show them partially filled such as using affiliate links, sponsoring ad spots in other contests or even rewarding space in contests. In the end it helped with nothing but some hiked Technorati and Incoming links values and nil in terms of monetization.
Now I think that its better to put something paid into empty spots than keeping them filled with unpaid and illusionary ad spots. And thus you are seeing Adsense on my blog back again. Even if it does not pay too much but something is better than nothing.
My Suggestion would be the same. Fill the unused empty space with something that’s useful for you too. Handing free adspots constantly won’t help much. Your residual income will still be zero. Don’t just use private ads just for the sake of it or just because everybody else is doing so.
And if you think you will never be able to get private ads by replacing the spots with Paying Ads like Adsense, you are wrong. Try to get more traffic(not links), and keep working. When Adsense starts paying you well, you will automatically have a proof that your ad spots are working good. This revenue can be a big proof to show to others that you can handle private ads pretty well. You can email potential advertisers and show them your potential and ask for private ads and can always replace Adsense with private ads if the advertiser is convinced.
So, I don’t think bringing back old forms of ads is bad that much. Adsense is still the easiest possible source of residual income online.