The problem with that is that leftism and Islamism can only co-exist to a very finite point. Islam doesn't even accept the basis of civil society---everything, including government, is measured in terms of "Allah's will." So while you may be right in the short run, that's all it can remain . . . a short run. 30 years is a short run? They will turn on each other when we fall and not a moment before. In the mean time they hold that "the West" is the source of all their evils. Common enemies.
In an ally, consideration of race and creed are insignificant beside two prime questions, which are:
1. Can he shoot?
2. Will he aim at your enemy?
The answer here for them is yes and yes.
Well, yes. 30 years in historical terms is a VERY short run. The entire Cold War lasted only 40. And I think that the schism between leftism and Islamism will be far shorter than that. Just look at today's news: several journalists in Jordan imprisoned for "blaspheming" Allah. It won't take long before these to isms are at each other's throats.
In the meantime, we must take care of ourselves and crush whichever one comes against us.