Question

Why do some anarchists continue to organize with Marxists despite their irreparable differences?

Answer 1

anarchists organize with all kinds of people, mostly for some combination of the following reasons:
a) they think that numbers are important for winning some limited goals.
b) they think that they will change people's minds in the process of working with them.
c) they think that people are complicated creatures who may believe some stupid things but have other characteristics that make them worth working with.
d) they don't realize the depth of the differences that anarchists have with most marxists.

in addition, some anarchists have crushes on marxists in particular because those anarchists are impressed by the rigor (practicality?)  that some marxists show in their thinking, and other anarchists will work with anyone who calls themselves revolutionary or who will join them on the street.

Comments

I gave an upvote because some of these are things I have literally heard some anarchists say.

Although, to bring up one possible explanation not mentioned here: I have worked with the occasional libertarian/anti-authoritarian Marxist on the basis that we happened to be close personal friends. But that might be considered distinct from Marxists without any other qualifier, who tend to be authoritarian communists, or "Marxists" as in Marxist groups and organizations.

Answer 2

For libertarian Marxists, they are often anti-authoritarians and if they have similar desires as I do, I'll work with them.  I also like having discussions with libertarian Marxists and can reciprocate knowledge with them.  

However, the wording on the question suggests organizing with Marxists that anarchists have irreparable differences with.  This suggests working in front groups, workplace committees, unions, protest organizing groups, academic conferences, Occupy assemblies, etc.  Here I would say "organize with" might be done..:

1. ..out of a naive thought that anarchists are leftists and all leftists should be able to work together on common purposes, despite their differences.  

2. ..in the chance that anarchists can convince the Marxists that the anarchist method of doing things is better.

3. ..in competition with the Marxists so a broad group doesn't fall to authoritarian machinations.

4. ..to ensure opposition to Democratic party proponents, liberals, progressives and other reformist elements so a more radical position can be held within a group.

5. ..the larger group is believed to have potential and attempts should be made to keep working within it despite differences.

Obviously other reasons can be considered.  Should the question be reworded to where we can assume the Marxists are an anti-authoritarian or at least not have irreparable differences, it can be assumed anarchists would work with them as they would any other individual or group.