> If you pass an object by value what you are doing is creating another copy of that instance that is seperate from the original instance. So if you work with a copy, then you need to sync up those copies at some point or some other such extra work.

Could you provide an example of this?

I'm a fan of being explicit about mutation and restricting possible scope of change. I think passing by reference isn't worth the hazard it presents, at least in a high-level language.

If everything has pass by value semantics, I can be confident in the state of an object, even if I pass it to other methods; it will never change unless I explicitly assign to it.

If globals are generally a bad idea, I don't see why pass by reference should be any different. I think the latter is a restricted from of the former, and should be marked explicitly -- here be dragons.