Picea omorika is a fine understock for grafting spruces, it's less common used then Picea abies or Picea pungens understock.
However I would use Picea pungens at the last place...

coniferjoy, I am curious why you would use Picea pungens as last option?

Coniflora, this is because they contain more resin then the other 2 which will make the grafting work more difficult.
P. pungens needles are much more pricly which also will make the choice very easy...

I guess it isn't used mostly because Picea omorika seedlings are too valuable as Picea omorika seedlings ;-)

Compare their prices with Picea abies seedlings of the same size, you'll see what I mean!

Resin

I use pungens - it's our state tree!