But a lot of great software is available as 'download only' too, in which case I'd rather the money all goes to the dev than to Apple, so I'd rather buy straight from the dev.

It's like Abebooks vs. Amazon - book sellers have to pay a larger cut to Amazon, so I get my 2nd hand books through Abebooks to help support 2nd hand book sellers.

Devs will have to make their own decisions about the margin cut vs. the potential 'exposure' of having their apps on an app store.
If it's brokenless, don't suffix it...