Originally Posted by dera

You can intercept it, just don't use green needles as course guidance.

It's even in the book. "Use the PREV function to capture the ILS course while still using FMS as the active NAV source".
If CA manually flips to green needles 25nm out, well, that's his call. It's just kinda pointless.
Also, in FM1, "APPROACH mode should not be selected until the aircraft is within the ILS service volume (approximately 18 NM of the threshold)".
And yes, I've been to ORD, nothing there prevents you from flying it correctly. Main point being - don't go green 25 miles out. The signal and obstacle clearance is not guaranteed, and it's sort of against the book too.