If I recall correctly, heavy use of Flash Heal was the thing in vanilla WoW when Molten Core was the place to raid. I hardly ever played as a healer back then, so I might be very wrong. Anyhow, Flash Healing was not ranked very high in TBC, but now in WOTLK (3.08) Flash Healing seems gain ground over Renew.
What one should use though is Glyph of Flash Heal and take the talent Serendipity. So when topping of your raid with Flash Heal (as might have been done with renew before) it is actually beneficial to overheal as you get a nice amount of mana back making your HpM (Healing per Mana) even better.
What's even better is that your Flash Heal can also trigger Holy Concentration and Surge of Light (if talented) , giving you even more mana and free heals.