I think the Market Bridge design is brilliant. But I don't see the need for it on the Willamette. The view crossing the Willamette downtown is one of Portland's most striking visual panoramas. The presence of shops on the bridge would only compromise this view and distract pedestrians and cyclists from some of Portland's most compelling scenery.
The Ponte Vecchio approach makes the most sense when trying to draw pedestrians across dead spots in the urban fabric. The most obvious candidate for this treatment is I-405 between Downtown and Goose Hollow. I can imagine market-bridge-style caps on several streets that cross the 405. These have the potential to dramatically re-connect Downtown with Goose Hollow and to catalyze development in the West End.
Below are some views of the "I-670 cap" in my hometown of Columbus, Ohio. It spans what was once a long, forbidding, wind-swept freeway overpass. This rift in the urban streetscape had created a formidable pedestrian barrier between Downtown Columbus and the rapidly-developing Short North neighborhood. Completed in 2004, the cap has worked an urban-design miracle and North High Street now forms a seamless, pedestrian-friendly corridor where there had once been a steel and concrete canyon.
Picture something similar in Portland linking key downtown streets across I-405 to Goose Hollow.