Which is probably what you meant, but your phrasing was (to me) a bit off.
However, targeting certain demographics by way of traditional advertising, like paying to interrupt everyone watching a broadcast television station, can suck as bad or worse, depending on the demographic.
As far as I can think of, it comes down to a couple of decisions: how much do you believe the medium, and not just the content, is going to affect the number of hits you get; and, how much is the feedback data you gather from your medium going to help your advertising campaign become more successful. There may be fewer people on Hulu, but your television does not provide a "thumbs-up" button at a per advertisement granularity.
So, while I think that the metrics would be bogus for trying to get a representative sample of a collective demographic who might watch motion video on any medium, I also don't think there is no worthwhile demographic information to be gleaned from posting one's media content on Hulu. It feels like a game of tradeoffs, and one you can probably only make an informed decision on by doing test runs, and seeing what data comes back.
Hopefully, this doesn't seem too pedantic; I just thought your assertion was too broad.
If pilots were all released on HULU, then a season filled with middling to poor pilots would be reviewed as such. And then no matter how much a studio might try to improve the best of the lot, they'd have those negative reviews dragging down performance of the ultimate product.
If pilot feedback is reasonably controlled, the studios have the option of polishing an almost-there concept (or more cynically: the best turd of the bunch) and not having reviews of its dodgy pilot dragging down performance of the final show.
This also means you can't manipulate the results by dumping shows in awkward time slots (interpreted another way, you avoid having to match each show to a time slot when its potential audience is likely to see it).
They can also measure things such as: what percentage of people decided to stop watching halfway through.