If I counted right, there are 5u, 8c, 7r destruction cards available at the moment. Regarding, that playing several commons after another doesn't have any effect and regarding, that the chance for drawing a common is almost 2/3, it should be no problem to add further common destruction cards... They are not needed, to increase the keyword-power IMO.*
Greetz, HivedOne.
*(some math for the weekend ;-)
P(c)=5*0,65=3,25; P(u)=8*0,29=2,32 P(r)=7*0,06=0,42... so P(c)>P(u)+P(r).
In english, even with all destruction cards in game, you should get more commons then rare...
should there "accidently" be only rare and common destructions in hand: play them in a row... and see your opponents ressis. melting ;-)