Sure. The illegal sites also need ads to cover their expenses - every site has expenses, be hosting, staff and so on. But they only have a couple of annoying pop-up windows which you you can close, a very user-friendly and easy cartoon image captcha (unlike Viki's captchas which are real-life photos, often a bit blurry, not good if your eyesight is not perfect, and not one but two or more) and then some picture ads which you can ignore. Once you start watching the video, there is nothing to distract or annoy you for the whole episode.
Viki of course has many more expenses, the licenses, so it is understandable that it needs more ads. But, as others have rightly said, REALLY OVERDOES IT. Not only they interrupt the video on the most crucial moments, but they are always the same ones, sometimes the same one two or three times in a row. Just like on TV! It is really saying "if you don't subscribe (or volunteer), you can go hang yourself, you'll have to be endlessly bugged until you do".
Of course, this policy works! That was the reason I started volunteering in the first place.