I can see both sides to this and maybe 'routing' is the wrong word but the sentiment is clear.
From a home user view with one Internet IP then therefore one TCP port 80/443/etc then there is at least one good reason to do reverse proxy on the SRM router: VPN Plus' SSL-VPN can't be reverse proxied via the standard DSM method (not tried anything else, as the clients complain that the server they are connecting to aren't VPN Plus.
...OK second reason: you don't have anything else that can do reverse proxy, at at least nothing that has a GUI/portal to make it easier.
The rest of DSM (and other) web packages that use Application Portal ... do they health check the web server is the right 'fingerprint' too? I don't know but if they work the same as VPN Plus clients then these will reject an SRM proxied TCP 443 ... and there's a lot more of them to break.
A limiting factor regarding SRM as reverse proxy is that the router support one certificate. Unless you've got a wildcard certificate then you'll soon run out of Subject Alternative Name characters (due to the 250-ish limit set by Synology) for all the proxies applications of Application Portal and Web Station.
Depending on what a reverse proxy is doing then these used to be very expensive bits of kit where the same Internet access would have a much larger reverse proxy appliance to the one that handled forward proxy. I'm not sure that the current SRM routers would have enough oomph to run anything but a very light r-proxy and, except the absolute lowest spec, the DSM NAS range has more RAM and CPU (and SATA disks).
To add reverse proxy to SRM isn't just a matter of adding that feature there're a raft of other changes that are needed to make it a suitable alternative to the DSM offer. But there will be some use cases for SRM
