BSG hovers around an (adjusted) 1.8 rating on Sci-Fi. Heroes has done over a 10 rating on NBC for the first two episodes. And yes, that many more people could see BSG on NBC than did on Sci-Fi. 1) There are still plenty of people in this country who have neither cable nor satellite, some by choice, some because they aren't an option where they live. 2) Not everyone who has cable or satellite gets Sci-Fi. In some cases cable companies simply don't carry the channel, so it isn't an option, in other cases it is not part of the basic service - it is bundled with other channels for an extra fee and that turns people off. Also Sci-Fi tends to be one of the higher-numbered cable channels on most systems, and therefore gets less "flipping around the dial" traffic. (Most people who are start on channel 1 will hit something that looks like its worth watching before they get to channel 118 or wherever they've stuck SFC. So they don't snag a lot of viewers not already familiar with the channel.)
Since Universal/NBC has been doing well enough (between ad rates, overseas distribution and DVD revenues) to make it to a third season, it probably isn't costing them too much per episode. So it is worth a shot for NBC, especially if a lot of the cost is going to by paind Sci-Fi and not the broadcast network's coffers. (Yeah, they're corporate siblings and all that, which is one reason that Heroes is running on NBC, Sci-Fi and USA network, but each entity is responsible for its own budget, so BSG could be worth sticking in a slot NBC is losing anyway if the cost per ratings point is lower than whatever they're running now.)
I wouldn't worry about Heroes or "ratings dilution". It had the best debut for a new drama NBC has seen in years. People watching on Sci-Fi and USA are either watching for a second time or they're people who are already watching and or TiVoing two shows during the Heroes timeslot and therefore wouldn't be able to see it at all. I watched the first episode on USA because technical difficulties with the sound on the hi-def feed of my local NBC station screwed up my HD-DVR recording. But given the choice, I'd much rather watch in HD so I'll be sticking with NBC. (The mere fact its in HD will also likely draw more eyeballs to BSG in key markets.)
Regards,
Joe