I am a great believer in getting bishops to wear as much as possible, especially the pontifical dalmatic, which should be worn under the chasuble. It reminds bishops they are also deacons, servants of the Church. It certainly should be worn for the ordinations of deacons.
I suspect originally the Dalmatic was an episcopal vestment, and was "given" to deacons as a sign of their sharing in the bishop's role as servant. In the East the sakkos, a dalmatic, is the main vestment of the Bishop.
NLM translates this liitle piece from the Office for Liturgical Celebrations of the Supreme Pontiff:
Bishops wear it in greater solemnities, under the chasuble, or even as a principle vestment in consecrating an altar or the washing of feet. In this last case, as the Caeremoniale Episcoporum 301 indicates, the bishop takes off the mitre and chasuble but not the dalmatic. It is desired to place emphasis not so much on the fullness of the priesthood as the character of service of the episcopal ministry. In the case of cardinal deacons vesting with the dalmatic, it serves to underline their character as servants, strict collaborators of the Roman Pontiff even in the liturgy. The dalmatic is a sign of service, dedication to the Bishop and others. But even when the bishop wears the dalmatic it is to serve: whether in the washing of feet, or in special liturgical service performed by bishops--cardinal deacons--in the presence of the Roman Pontiff.So, encourage your bishop to truly be a servant, get him a pontifical dalmatic to remind him he is not just hierarch!