New Photoshop versions introduced support for linked smart objects: http://blogs.adobe.com/richardcurtis/2014/01/16/photoshop-cc-14-2-update-linked-smart-objects/. Using linked objects is a good thing. First, it allows to share image assets between psd files. Second, it makes size of PSD file much smaller.Initially, linked smart objects were not fully supported by the automated psd to html conversion tool. One of the processing phases was skipped if PSD file contained such layers and warning was generated by the service. It was required to embed all the linked objects into PSD template. Although it may be done in one click, it’s inconvenient.
This limitation does not exist anymore. So, one can freely use external files as linked smart objects. In other words, users should not perform linked layers embedding by hand. Service will do everything automatically.