Have a bike right now with Ultegra Di2 disc groupset. Was looking at getting a backup bike that has Ultegra mechanical with rim brakes.

However, I've had a number of people tell me I'll really regret not spending another $700-1,000 bucks to get similar components on the backup.

The cross-compatibility argument sounds nice in theory, but wondering if anyone has any experience with how useful this actually is? Guess I could see it being nice having the ability to swap wheels in a pinch, but I feel like I'd just end up taking the other bike anyways if there was a wheel issue on the primary.

Is there something else I'm missing here? The backup will spend a bunch of time on the trainer with probably an outside ride here and there. Just don't know if it's worth the extra $ to keep components constant.

Thanks!