I'm going to try to express myself very carefully here, because this depends on a series of relatively fine distinctions, despite their validity, and apparently even obscure people on the left can be used to make a general smear (see Ward Churchill, all over the bloody place). Ted Barlow has a wonderfully titled post over at Crooked Timber, picking up on a right wing blogger claiming that the left is engaged in dastardly conspiracy with the despicable forces of Radical Islam to destroy all that is good and wholesome about the world. The left generally is clearly not in dastardly conspiracy with the despicable forces of Radical Islam: if they were, they'd be in prison, probably being tortured. The left - and I include myself here - does however dislike a lot of the things that US (which is explicitly what is meant by all that is good and wholesome about the world here) does, including in foreign policy.
A lot of the left - and there are exceptions: Harry's Place, for example - thought and continue to think that the Iraq War was a mendaciously justified, illegal, unnecessary, badly executed invasion, which will end up being completely unjustified, because its costs will vastly outweigh any plausible benefits. The left tends to think this about a lot of the foreign policy of the US, and in many cases, it's even clearer than in Iraq: the vast majority of its interventions in Latin America, for example. This means that the left can see some advantages to things which impose constraints on American foreign policy, things which raise the costs of foreign policy adventures for the political establishment in the US.
If you don't want the American corporate elite attempt to run erstwhile sovereign countries substanially for their benefit, then there is a prima facie advantage to things which make that more difficult. It is not necessarily an all-things-considered advantage, because there are other costs to be considered: the harm that could be caused to people, obviously including Americans, by the acts that make it more difficult to project American power around the globe, is an obvious disadvantage, which very often makes such acts just clearly vastly wrong in the all-things-considered sense. I seems to me that all acts of genuine terrorism have this character: terrorism, like torture or carpet bombing built-up areas, is just intrinsically wrong, and no matter what prudential considerations can be made in favour of it, it should never be done.
Still, to some degree, the prudential considerations remain, even though they should never be acted on. Thus, the left can see a prudential consideration in favour of Radical Islam insofar as it successfully opposes some aspects of American foreign policy: obviously, it's not a prudential consideration that means they support either the project as a whole or the means used to achieve the ends given by that project, since they abhor both the project and the means used to achieve its ends, but it is, to some extent, a valid prudential consideration. For example, assuming that the insurgency in Iraq is motivated by Radical Islam, which I don't think it is, if we also assume that once Iraq has calmed down, Iran is next on the list, and we think invading Iran would be at least as stupid and wrong as invading Iraq, the insurgency in Iraq is preventing that, which is something to be said for it. Again, obviously, the insurgents are a despicable group of people, with pretty uniformly awful aims and disgusting methods of attempting to achieve them, so the left generally despises them and certainly does not support them, but, the fact that they are preventing the US from projecting its power elsewhere is some kind of an advantage.
I've been quite careful to say that I don't support terrorists of any stripe, and that I abhor the aims and methods of Radical Islam, so, please don't accuse me of that. What I am saying is that, given that the left would generally prefer the US not to act in the way it does in foreign policy, things which make it more difficult for the US to act in the way it does in foreign policy have something to be said for them, specifically, that they make it more difficult for the US to act in the way it does in foreign policy. In lots of cases, that's all that there is to be said for them, and since these things are often fairly f*cking awful themselves, it doesn't really have any bearing on their all-things-considered moral status. But, in some restricted sense, it's something to be said for them.