FRIT (and Partners) have been significantly more open to funding transit than Wynn. FRIT plunked down $15M for the station (in addition to paying for some of the AS roads, if I recall), and assisted the MBTA with getting state and federal funding. The MBTA paid no money for the construction of the station, and because it was an infill station, the additional fare revenue likely more than covers the marginal operational cost. Partners is building 1600 parking spaces for 4100 employees, and is paying for the construction cost of the full south headhouse. And both of them chose Assembly partially because it is located on a high-frequency transit line with the ability for a stop to be located there.
Assembly is very probably a net transit benefit even for those not going to properties that FRIT or Partners are involved in; I'm guessing that the improved access to the Assembly Square Mall, Home Depot, Stop & Shop, the District Court, and the Ten Hills neighborhood outweighs the slightly increased travel time for existing Wellington-north riders.
Wynn chose to locate his casino in a location poorly served by existing transit (at least Suffolk Downs would have had the Blue Line), without the ability to add an infill station. He knowingly faked the traffic study (read the comments from Mikel Murga and Fred Salvucci on the DEIS - the methodology used is intended to analyzing intersections in isolation and is wholly inappropriate for closely-spaced intersections with the potential for interfering queues [i.e, anywhere in urban areas], and is pretty obviously meant to hide the potential for actual local or regional issues). He based the traffic study on ignoring preexisting plans for calming Sullivan Square, including removing the underpass.
Other than paying to extend rush-hour frequency on the Orange Line (which I do applaud), Wynn's plans only serve the casino and don't benefit other areas. I would rather have seen him pay for additional frequencies on the 104/105/109 (gee, isn't that how the Everett natives he claims he'll employ would get to work?) and perhaps create a high-frequency, high-reliability Everett Square short-turn. Pay for additional Silver Line buses to run off-busway from Chelsea to Sullivan, thus providing a one-seat ride from South Station and the Seaport and a two-seat (or a new one-seat) from Logan. At least fund a study to see whether it'd be feasible to extend a third GLX branch from the carhouse leads, up the abandoned yard tracks past Sullivan, and to the Casino and downtown Everett.