WinSCP puts saved files onto transfer queue (upload on background connection is just a side effect of this), because as it allows you to edit multiple files in an external editors, there's nothing to prevent you saving another file while WinSCP if uploading the first one.
I understand your original intent and it seems good. Yes, WinSCP does allow editing multiple files at the same time - in theory. But do people really do it?! I just got 1 head and 2 hands - hence - yes, I can open multiple files - but in practice I only edit one at a time, even if more are open. How on Earth could I save multiple files one after another?! Editing takes time - much longer than uploading. So the result of this good intent is much slower upload which renders the whole remote editing feature unsuitable for me.
Anyway - even if some people do need this feature - can you add an option to save remotely edited files via regular upload? I guess it's not much work.
Can background transfer use the existing connection (option?) to avoid the overhead of connecting/disconnecting just because for a single file? You may object again that in that case I won't be able to up/download files the usual way until the transfer is done - but I WANT THAT BEHAVIOUR. Otherwise it's ridiculous: WinSCP already has an open connection ready for use, but in order to upload a file it opens another one (while the first is not used), and then closes it as soon as the transfer is done! I guess it would make more sense if WinSCP opened another connection only when the first one is being used at that very moment. It shouldn't even depend on whether the transfer is in background or not - WinSCP should have a pool of connections available to both modes, right?