Maybe they're trying to appeal to people like myself. I always enjoy new material, whether or not I ever get the chance to use it. The principle of the matter is when I really do need it, I know where to get it.
I own almost all WotC books released (soon to get d20 Modern books next) for D&D and there have been books I never cracked open---BUT should the time come when I do need to build an adventure or campaign on illithids or freaky aberrations taking over, I can open up my Lords of Madness with a smile. Or if I need more dragon material (Draconomicon), want a more "horror" feel to the next quest (Heroes of Horror), staging the next huge adventure around Incarnum, or even if my players want to try something new, I have that material at my disposal when I am ready.
Who knows when I will need an NPC, or if one of my players ever chooses to play, to be a halfling whisperer or diamond dragon? I know if the inspiration ever hits me (or a player), I have that material available.
It's really about availability for me. And probably for others, too. I, for one, am a little excited about this D&D Insider, but I do need to know more information about this before I canonball into it.
Maybe 1 or 2 months free when you subscribe and you can cancel it if it's not to your liking? I think that'd be the smart way to go about this to please everyone.
Another thing people need to realize is there're gamers out there that would love to see a chunk of new material for their Spellthief, Ninja, or Hexblade. New feats specific for their Beguiler, new epic material (that we haven't seen in ages) for people that run epic games, incarnum material for those that use Magic of Incarnum heavily, and others. But would you rather see WotC do a book on material used by the minority, or would you rather them please the majority by placing the minor material on their website but for price while continuing to do their sourcebooks on more bigger topics?
That way, everyone gets what they want.