Have you ever had an MX site that is down remote to you - and because you can't see anything you can't be sure what is wrong?
I always thought it would be a great feature if you could put an iPhone or Android phone into Hotspot mode, and plug it into the USB port on the MX to give it temporary Internet access (like a 3G dongle). Then you can check the WAN circuit status, event log, etc.
In every MX we are using a Verizon USB card with an antenna that I've tried to mount as high up as I can go. All of the cards are on a M2M pooled plan so it doesn't cost a ton each month to keep them active. The backups have helped me many times in the past when a circuit has went down for maintenance or network issues. Some sites I ran solely on a cellular card until the fiber circuit was dropped in and ready. I only have one site that needs the antenna mounted higher as the cellular connection is a bit flaky.
It is nice too if a MX flips to cellular, I know it's a circuit issue, but if the MX goes hard down then I know it's a power issue or in one case specifically, the MX kicked the bucket. RIP MX70.