Clients may not care what you build their apps with as long as it is sound and you use accepted practices (And some don't even take the time to check any of this.). However, if they are under the impression that apps can be built quicker with this type of product, more developers may be pressured to start using it.
You may find yourself with the task of taking over an existing Lightswitch application created by a power user and they need more functionality. Supposedly you can still work with this in Visual Studio to do more.
If there is limited programming expertise in a company, there may be a need to let some of the power users to take over app development. You're more of a teacher/facilitator than purely a programmer except for the heavy lifting. This is common with report writing and may be expanding as rapid application tools improve.
Other than this, I don't see any reason to put too much energy and effort into this if you are already using Visual Studio.