IGMP report messages..

Hi,
I have doubt about IGMP report messages.
IGMP report messages are sent to group-address.
Is there any reason why we don't send messages to
ALL_IGMP_HOSTS ? (at least once in 100 sec.). So
that routers can learn groups dynamically.

As of now most router implementations does not accept
packet from any UNKNOWN group. So it can accept packet
only if router has configuration about possible groups
in its network.
Many implementation does not support Router Alert Option as
well. So what is the soultion for this ?

with regards,
Ajay Thakur