The skill is useful but only to a small degree. This skill is only meant for beatdown decks. It is comparable to Banner of Courage and Graceful Dice due to the small ATK boost effect for every monsters on your side of the field. The biggest boost that you can get out of this skill is 300 ATK per monsters and only on your turn. Since this skill can be used once per turn, you can activate this on each of your turn to get the ATK boost after you have finished summoning your monsters. However, the fact that the ATK boost is not very significant still stands.
So I just figured this out. I had 2 monsters on the field, I summoned Winged Minion, activated the skill so each monster got 300 attack, then activated Winged Minion's effect of adding 700 attack to any Fiend type monster I have. The 300 I gained from this skill did not go down to 2 after Winged Minion left the field, so if the number of monsters on your side of the field will decrease before battle for some reason (like with my test) be sure to activate this skill BEFORE those monsters leave your side of the field. Union decks seem to good examples of this