I often open up "scratchpad" text documents. I Paste some text, edit it, Select All, then Cut. I press CMD-W to close the window. Because the document is blank, I'm not prompted to save the unsaved File.
If I try to do the same thing with Soulver, when I close the (blank) window the app pops up a modal dialog asking me where I want to save the (blank, never-saved, scratchpad-only) File. So annoying.
It seriously prevents me from quickly opening up or using Soulver just because I don't want to deal with clicking through that modal once I'm done. I'll just use a Text document instead or put the math equation into the Chrome omnibox.