many people get confused about the main purpose of price feed mechanism.

Let's make it clear:
The present of price feed is NOT to mimic the STEEM/USD price at external market,
but to give a suitable STEEM/USD rate that could help maintaining the SBD-USD 1:1 peg:

The primary concern of Steem feed producers is to maintain a stable one-to-one
conversion between SMD and the U.S. Dollar (USD).

This sentence is not mine, but an excerpt from the White Paper. (page 14).