I think it comes down to total cost including the cost of these “appliances” compared to the cost of upgrading (or waiting to upgrade) to versions of vendor products that support the needed security capabilties (WS-Security, SAML, encryption, signature, etc.).
If you are faced with an alternative involving lots of custom development in order to meet security requirements vs. overcoming some potential organizational issues to plug in and configure an appliance, it may be worth the brain damage.
Besides, we have to work with network support folks already to configure firewalls, proxy servers and load balancers. This is just one more piece of hardware although it does overlap with the software space more than those other examples.
I once used (OK, was forced to use) an “integration appliance”. It was a rack mounted device and you developed integration logic using a web-based application.
It lacked many capabilities of webMethods IS and never really caught on, but it was a much lower cost item and might have been well-suited for simple, departmental integration challenges.