Because you obviously were mixing pixel scraping (used by RDP) with pixel pushing.
Quoting yourself: "Why? RDP also pushes pixels"
Doesn't it push the pixels over the network after scraping them?
You were also obviously confusing using Xrender to push pixels and using Xrender in a way which leads to low bandwidth usage.
Again, quoting yourself The term "software rasterizer" does not necessarily imply that it does not use Xrender.
Which is true. I don't see any confusion here on my part. Pixels can be pushed over the network and used in different ways on the server usind Xrender or not. Exactly as I said: the term "software rasterizer" does not imply exactly what is done.
Using the X protocol for a screen scraper is a bad ideia:
a) it does not provide means to compress the image on the wire.
This is true, but if used with ssh -X there would be some compression at a lower level. Also adding an extension to transfer compressed images would be very easy.
b) the display X server won't always keep the contents of the window. Eg, if you minimize and then restore a window, the display X server may require the contents of the window to be re-send, although they haven't been changed by the application.
Yes, but the application can store stuff at the X server and then later access it. I know, I wrote such code.
I really can't fathom why you insist on using the X protocol for a pixel scraper. There isn't anything new here. We have a bunch of pixel scrapers around for a long time (VNC, RDP, Xpra and, to some degree, NX) and they've all found it useful to use a specific protocol.
Because using X would backwards/forwards compatible, cross-platform, widely supported on UNIX/Linux (so far), and effortless (just use ssh -X). It also would keep the door open for applications which want to do something more clever than pixel scraping.
Some people like RDP because despite being owned by MS (and subject to patents) the FreeRDP project provides a good implementation of the RDP protocol, which AFAIK, compares favourably with other pixel scrapers.
Last time I tried to connect to a Windows server from Linux it simply did not work. But I am not opposed RDP. But I like X more because of the aforementioned reasons.