I have seen some DIY jobs that use the faucet, and the reason I see them is because we are there to completely re-do the system after a new home owner moves into the house.
I also see a lot of pool companies tapping into the faucet and installing AVB's to use it as the pool auto-fill. This is mainly because recent regulation changes prohibit pool companies from using the irrigation system as a auto fill source.
I would not expect any more than 3-5 gpm (depending on pressure) out of a faucet. Also- If they have a water softener and the faucet is in the "loop" they will be re-filling salt on a weekly basis.
There are a lot of negatives, why can't you tap into the supply before it gets to the house?