The big scare factor in TEM is international roaming charges. One vendor relayed me several horror stories of large companies wasting tens of thousands of dollars a year as employees ran up huge roaming charges when abroad, out of ignorance. The answer of course was to implement the vendor's TEM-oriented MDM tool. But the numbers rarely add up. Per-device license fees run $60 to $125 per year in most cases. If you have 5,000 employees (the number cited by this vendor in its examples), you're shelling out $300,000 to $625,000 per year (assuming they have just one corporate-paid data plan each) to avoid anywhere from $30,000 to $200,000 in unnecessary international roaming costs. That doesn't make sense, does it?
What would make sense is if you are employing MDM anyhow for its other management and security capabilities on all those devices that you favor providers whose MDM tools also check roaming status and send the users a note telling them to turn off data roaming when abroad. Or you might negotiate international roaming rates with your carrier if you have that scale of users on their network under a corporatewide plan. But citing international roaming costs as a reason to do MDM is silly. That can be a fringe benefit of MDM, but not its justification. And remember: These MDM tools can't handle your carrier contracts or monitor your "ghost" devices; you still need to do that as part of your purchasing management, with or without a TEM cosultant.
If you have BYOD users in addition to the corporate-managed users, the MDM tools apply to the BYOD users' devices, but of course your carrier contracts and TEM activities outside of roaming monitoring don't apply. As the whole point of BYOD is to let users lead on technology, they'll pick the data plans that they want to pay for, and you'll have (or should have) a policy as to what you'll reimburse (if any) that ensures you're not payng more than you choose. It'll take one BYOD user who gets a $3,000 bill for roaming when in, say, Japan to make it clear where the responsibility lies.
In fact, that policy should apply to corporate-managed users. Why should their department pay for out-of-policy expenses? That will quickly lead to the behavior you want. I find it ironic that so many IT pros and purchasing managers want to automate away the users' responsibility, then get upset when the users act irresponsibly. If the bills are invisible, how would they even know they're wasting money?
And don't forget the low-tech approach: Don't issue international-roaming-capable devices to your employees who don't travel abroad; instead, have some loaners for those who do occasionally. I realize that the iPhone 4S now offers international roaming as a standard, but older iPhones (which are still sold) do not, and many Android devices are not world phones.
You really need to understand the total cost of management, not just the cost of nonmanagement. For example, maybe only 500 of the 5,000 employees need data plans for work. That means you don't have an MDM tool for 4,500 of them to check if they're roaming abroad and will need to rely on a corporate international roaming plan for them and your 500 data-using mobile employees. Maybe a separate MDM tool is too pricey for just 500 users, so you rely on Exchange for the management and security and handle these 500 as "regular price" corporate-managed devices or as BYOD users who get a stipend for data service each month. You rely on education and nonreimbursement of excess costs to manage their international roaming charges and "ghost" device charges.
If you have hundreds of mobile employees
Most companies don't have many thousands of employees. They have dozens or hundreds, maybe a few thousand. But the vendor stories out there are about companes with tens of thousands of users, where it makes more sense to have central management and oversight (whether in IT or purchasing) given the scale of the deployment.
Of course, you have the same options as the larger companies that I just described. But your per-seat MDM licenses will be higher the fewer seats you have, and you'll get fewer discounts from your carrier for corporate-managed employees. The large-company approach simply may not work for you.