I have MMShareFindPboard set to false so that MacVim's search string doesn't interfere with that of other Cocoa apps. (There's nothing worse than e.g., hitting Cmd-F in Safari only to find the little search box pre-populated with something like \(foo\|bar\)\+.)

I've noticed however that MacVim will clear another app's search string.

Reproduce: Open Safari, go to some webpage, and search (Cmd-F) for some string that is present on the page. Maybe hit Cmd-G to go to the next instance. Do not close the "find bar" in Safari. Then Cmd-Tab to an open MacVim document (or just open one afresh) and hit /; enter a search string and hit . Then Cmd-Tab back to Safari and observe that, while the find bar is still open and occurrences of your search string are still highlighted, your search string is gone, and Cmd-G no longer works because it doesn't know what string it should be looking for the next occurrence of.