Can vendor lock-in ever be valuable?

You should be considering if not planning the exit strategy as you are evaluating the new vendor/solution

"Lock-in" is as much about perspective as it is being beholden to a villainous vendor.

Poorly managed infrastructure and app strategies are a lock in risk, it's just lock in to inflexible and potentially expensive "do it yourself". 

Lock-in to a major vendor can speed time to value for changes and refreshes. However, you are locked into a single supply chain, which comes with it's own set of risks. 
- Cost 
- Innovation (or lack thereof)
- No competitive differentiation
- Etc. 

To know you have lock in is key and to plan your organization and architecture around the type of lock in you're accepting is key.