4 creeps every 30 seconds is 8 creeps a minute? +3 siege weapons, which helps a little
Creeps also average around 45 gold per kill, if you can farm 100% of the wave every time you can get ~400 gpm with no kills or towers.
And yes, the point of discussion is to say that BF is not a great item for the cost and you should not buy it unless you farm is very, very near perfect in early game and you can use it to snowball into 30 minute manta/heart. If you get your regen ring at 3:00 and get ganked twice and aren't doing so well, make a vanguard with it instead because a 20+ minute BF will have you sitting at 800 health when team fights start happening, your choices will be to force your team to 4v5 (lots of pubs do this unfortunately) or to jump in team fights and die instantly.