I've never had a problem telling the teammates apart.
Still, roughly half the drivers who are currently in F1 has a helmet design with a blue top. That, and the sponsor band, is usually all you can see because of the high cockpit sides. I agree that in most cases it's no problem telling them apart if you follow F1 closely, but I can think of plenty of examples when it wasn't easy at all. For instance, Schumi / Rubens (before Schumi changed his design), Panis / Trulli, Alesi / Trulli, Rubens / Magnussen etc. This year I imagine Trulli / Button and Fisichella / Sato won't be very easy to tell apart, and there are a few others I'm not so sure about.
Besides, clearly visible car numbers is a convenient way for commentators to tell viewers who's who. Compare:
"That's Webber in the 23 car and Yoong in the 22 car."
"That's Webber in the black car and a blue helmet with red and white stripes, a yellow top with greenish stripes on it, and Yoong in the black car and a red and white striped helmet, except it's yellow on the lower part, and has a blue area with sort of a yellow star-shaped thingy on top."