Aggregates and Indexes

Their has been some discussion regarding aggregate functions and indexes.
It's clear that aggregate functions will not take advantage of any indexes.
(ie. SELECT sum(number) FROM table WHERE type=BLAH will not take advantage
of 'type' being indexed.)

I do a lot of reporting based on such SQL statements. Counting rows from
large datasets. Since the PG gurus don't seem to think this is such a big
deal can someone enlighten me as to why?

Thanks,
Adam Erickson