Quickening is simply a cheaper version of other ways to keep the buffs active, with downside that it can be broken and cause a permanent loss of Karma. If that downside actually features, people will need to start weighing their options. If they never feature the downside and/or can hunt for those massive hits (which is why usually people mandate buying hits), it becomes overkill. Don't forget that to subtly quicken spells, you already need 3 Metamagics. The SR4 cost for quickening was way too high given the risk involved.
If, as a GM, you are sure you are not capable or willing of featuring the downsides of Quickening properly, just ban the metamagic from your game and save everyone a headache. The mages can still get sustaining Foci or Focused Concentration, take the penalty with Psyche, or have a Spirit sustain the spell on them for a while. If you are going to use dispell and wards well, then all you need is 'can only quicken spells that use bought hits' to prevent cheesy abuse.