Those small boxes are made for older analog sets at present and do not let anyone view in HD which is what the HDMI cable/output is usually for. Those present DTAs(small boxes) are made for receiving the expanded basic/digital starter channels max, no HD or Music channels. If you require HDMI on your other tvs you will need full HD set top boxes for each tv.
There are rumors of an HD DTA , which should use HDMI and/or Component outputs, that may be coming out in the future.
When Comcast rolled out the digital age transfer to customers they offered two small adapter boxes (free) for use in other rooms which I have. These in addition to the main box in another room. Does Comcast offer these small adapter boxes w/ HDMI out?