Hazlewood and Cummins are not Test specialists. They are both fixtures in Australia's first-choice ODI attack. In fact, Hazlewood (5th ranked at the moment) was the top ranked ODI bowler last year if I am not mistaken.
Siddle was dropped early in ODIs because he was very poor. The only reason he is playing this Test series is because Hazlewood and Cummins are not there, and he can bowl long spells.
If the likes of Amir and Hasan were taking wickets, there would have been no point in testing Abbas in ODIs. However, the former two cannot buy a wicket to save their lives, and Abbas cannot stop taking them these days. Pakistan should cash on his great form.
If he can nip the ball off the seam with the length that he is bowling, he will be a real handful in ODIs as well and will be hard to hit. However, if he fails, he fails - not a big deal, it is not as if our current bowlers in ODIs are not failing anyway.
I think he should play at the World Cup and bowl 7 overs up-front. If he can do that by taking a couple of wickets and economy rate a shade below 5, we can restrict teams below par totals. Amir cannot take wickets and Hasan gets smashed when he cannot find them, and perhaps Abbas can do both.
It is worth noting that we have no chance of winning the World Cup, but that does not mean that we should not play our best playing XI.