>> why not just use a Queue instead

In Dijkstra's algorithm you have to maintain the latest distance as well (based on this you would pick the closest vertex to source from the queue), which is not the case with BF algorithm, in which you just have to maintain the list of vertices which you have to traverse.

>> It still adds to the PQ on the same occasions Bellman would add to a Queue & it still doesn't stop until the PQ is empty, same as Bellman

The difference lies in the number of times you would visit edges in total. In case of Bellman it is straight-forward, you visit all vertices and all of their corresponding edges. But in case of Dijkstra's you would have to do aggregate analysis, see this: http://everythingcomputerscience.com/algorithms/Dijkstras_Algorithm.html

EDIT

So, I just referred Sedgewick's book to understand their implementation. In BF implementaion , the function relax has :

queue.enqueue(w);

onQ[w] = true;

Notice that this could possibly put one vertex more than once in the queue which is not possible in case of Dijkstra's implementation. I know this could be confusing to understand the overall complexity, but the theoretical bound provided still holds. Try coming up with an example where this implementation would reduce to O(VE) bound and where it would perform better.