The answer to this question largely depends on the nature of the business. It depends on how sensitive the information is that employees are exposed to, the maturity level of the employees, and whether or not the work involves collaboration with others or quiet focus. The answer depends on your organizations' risk tolerance and productivity goals.
Security as an Issue:
Sensitive Personal Information:
If you work for a company that deals with a lot of very sensitive data, then bringing in any device that could be used to compromise the security and privacy of the data may fall strictly within the bounds of being completely restricted.
Cutting Edge Technologies:
Or, take Apple for instance, whose employees regularly work on cutting edge technologies. An organization such as this may also have a need to restrict the usage of personal devices on company property or even make employees sign NDA's so that they are legally liable for damages if sensitive information is leaked through the actions of that person.
Nature of The Work:
However, if you're working somewhere where the sensitivity of data is not as important, then the question of "information leaks" may be a non-issue. Instead, employers may only need to focus on whether or not the devices are disruptive to the workplace.
For instance, if employees regularly play games on their phone or if their tablet rings everytime they get a call through their Skype mobile app, this could have an effect on productivity, especially if there are workers who need a quiet work environment to be effective. An employer may need to set guidelines restricting the use of the devices or how they're configured so they're least disruptive.
However, in other cases, such as in a sales role, having access to such a device could make it easier for a worker to update a social network or contact a client in another country using Wifi calling or instant messaging. In these cases, employers may want to encourage the use of such devices.
It Depends On Risk Tolerance:
I've worked in places that would restrict such devices and require key-card access to the building, due to the sensitive nature of client data. Conversely, I've also worked in places where intellectual property was a non-issue and where management was very hands-off. Some employers have even provided such devices for both our work use as well as personal use.
In short, your policy depends on how much risk such devices have on your business or your clients' businesses, and what type of impact it will have on your employees productivity, whether that impact be positive or negative.