EDIT: for example, Poloniex doesn't need an XMR GUI to offer users a robust and intuitive crypto exchange... and XMR.TO and Shapeshift don't need an XMR GUI to offer users quick and easy btc conversion.
Except that is completely wrong. Poloniex can't offer a robust and intuitive crypto exchange for XMR unless users have the ability to
withdraw their coins in a manner consistent with the skill and comfort level. The same applies to all of the others more or less (less so for MyMonero).
It is also the case that spending coins left on an exchange, while possible, is inconvenient.
Part of the appeal of a private store of value is that you can actually store it yourself under your own control. Only then can you proceed to use it by spending it (which in turn makes it attractive to offer services to people using Monero as a private store of value), but not before. Without a GUI many potential users can't do either.
I'm not sure exactly what you're disagreeing with. I'm not saying GUI's aren't important. I'm saying they're products that should be left to third parties and are largely outside the scope of what the core software should include.
