Boeing recommend you don't use flap as an extra speedbrake.
Funnily enough most companies repeat this advice in their ops manuals. This is to prevent damage to the flap tracks and other bits. A flap track failure at worst may prove uncontrollable.
So do you follow the advice of the manufacturer or your mate on the flight deck ? Seems like a no brainer to me. Use the flap as the manufacturer and your company recommend. To do otherwise is poor airmanship.
Exceptions would be situations requiring you to get on the ground asap. You then can use the full limits knowing that nothing will fall off.
As pilots we often complain when the company rosters us to the edge of legal limits. We observe that while it may be legal it is most certainly not wise over a protracted period as sooner or later there will be an unpleasant consequence. Why is the aircraft any different especially as when we make the rostering complaint we ofton observe that if we treated the aircraft the way the company treats us they would break.
I do work for a large loco in Europe so do know what the pressures are.