I agree with you. While yes, a high powered media server will send more simultaneous transcoded streams to low powered devices like Rokus, I find them unnecessarily complicated and my QNAP 451+ does the media server job just fine. Of course, a mini powerhouse like a Shield does not require transcoded streams and thus have transcoding disabled on my Plex server. I'm not sure how well the QNAP's downloading app would meet TLS's downloading requirements but it's there and possibly worth experimenting with.
And yes a HTPC has access to some streams via web browser that are not directly available to a Shield, but fortunately, they are few and far between and nothing that I'm likely miss so I'll take the convenience and cleanness of a Shield's easy as a cable box menu driven app based interface over the clunky (for a HTPC) mouse driven interface of Windows or a Mac and their various browsers. Been there, done that, have bruises and the t-shirt.
The only thing that I really miss on the Shield is Amazon music. On the other hand, I have the Google Music app, Youtube app (4K), Amazon Video app (4K with HDR), Netflix app (also 4K with HDR), plus 1080P streaming apps for most premium and many basic cable channels, plus Plex and Kodi - meaning that I only need one HD FIOS box for the video content that I care about, relying on Shields and Rokus for other rooms and I receive 4K content from the very few sources streaming it. While I'll soon be buying a 4K Blu-Ray player, streaming is indeed the future.