Because it is immensely irritating to be much weaker for 12 seconds because I'm Sona and can't really control where both of my Q shots go to. Or maybe you just need to change Sona or something? I was at 3 charges and still had to wait because one of my Q bolts killed a minion before the other one hit an opponent.
I think the point of that item is, it gives you more gold... as long as you don't kill minions. Simple solution, don't kill minions. Complication, I play Sona.
Solution I: go with a different gold income item. Bit of a cop-out as an answer, but if you want a steady, guaranteed income of gold, maybe 
is more your speed.
Solution II: What Letler said. Just watch the minions and avoid Q'ing when low minions are near.
That gold income build path is intentionally risky because it has the potential to earn you made money as well as deal a good amount of bonus damage.
If you are playing Sona try not to use it when a nearby minion is so low on health you could kill it?
I would just rush the item as fast as possible. With the trinket system you can afford to delay the sightstone should you go that route, and as sona in particular you can put off boots.
Previously I wouldn't recommend this but the 10% cdr goes a long way to not gimping you, and it pays out dividends in the long term.
