Dedicated LUs simplify management, since the mainframe personnel always know who has access to which session. The disadvantage is that VTAM tables become very large. And every time a user is added, the tables must be updated.
Dynamic allocation insures flexibility and eliminates waste.
A mainframe gateway should support both methods. On the whole, mainframe gateways offer very little in the way of management capabilities. This is one area where improvement is needed. The gateway PC takes care of the basic communication and session handling with the mainframe. But each workstation must run software as well.