@Orange_Rouge Meraki Now can be worthwhile for customers who may not have highly critical up-time requirements or SLAs to uphold, and leverage Meraki Now strategically where doubling up lots of infrastructure becomes cost-prohibitive and a few hours of downtime is acceptable.
Like others have stated, if up-time is critical then you'll likely want to deploy warm-spare configurations, or at least have on-site cold spares. Or if you're running a wireless network with hundreds of APs for example, you'll likely have a couple shelf spares without the need for anything more than NBD hardware replacement because the wireless can be self-healing around a failed AP.
So Meraki Now seems to be most popular with MS and MX. Sometimes it's the NBD component that customers want to short circuit with Meraki Now... if a switch fails on a Friday morning before a 3-day weekend, you might get the replacement switch the same day instead of the following Tuesday. Sometimes it's the on-site support component that customers want to buy into, and the 4-hour hardware replacement is more of a bonus for them.