Actually it is defense in depth even though they are all contained on one appliance or device. Think about the layers in a bullet proof vest. They each work in tandem to prevent damage to the person wearing it. However just one type of layer by itself would likely not be enough protection against certain firearms.
Granted it is a single point of failure, but the ability to manage an entire suite of services from one console is attractive to many smaller organizations that may not be able to provide the care and feeding of single purpose devices. The ability of a vendor to patch the entire product suite against vulnerabilities is another good reason to go to a UTM device. If using multiple devices from different vendors, then the vulnerability exposure could potentially be greater if one vendor addresses a vulnerability in their appliance/service but another does not.
I would go to standalone devices if the potential threat to my organization could create capacity/performance issues on the UTM device.
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