There are two schools of thought about this. Mine is that you should only share work that is done, in "penultimate" form. Giving someone a "rough draft" verges on the insulting. Moreover, it puts the reader in an awkward position. Should I point out rather obvious lapses that the writer could easily catch herself? Or should I assume that he really needs help with some basic issues? I have to guess at what needs commentary and what doesn't. I don't want to waste my time with issues that the writer already knows how to fix, with the possibility of insulting him, but I don't know which is which. Does the writer have problems with organization, or did she give me something before she bothered to organize it? You should only share with me a smooth draft.
The other school of thought is that you need to share work sometimes earlier, just to get another perspective, or for reassurance. If I need feedback on ideas, then what I would normally do is to send a more informal email and lay out some ideas, to see whether they have plausibility. On beginning one of my current thoughts I wrote four or five people in the field asking their opinion about whether it would be viable, giving them a sketch of what I was planning.