If the Parish Council fosters a positive, constructive relationship with the District Council, there can be significant benefits in having a District Councillor on the Parish Council. It opens a two-way conversation and enhances understanding of the challenges each council faces. From the Parish Council's point of view, having one of your own sitting at the top table can only be a good thing.
With your district hat on, you simply need to be honest about what is and isn't achievable. In my district, around half of the district councillors also sit on parish or town councils and I am not aware of any significant problems over the last five years.
We do have a neighbouring parish council that considers the district council to be the enemy in a battle for resources, so it would be difficult to see how a district counicllor might fit in there.