Actually, there is more demand for concert hall space than you'd think. Jordan Hall is both very expensive to rent and so frequently in use that it's surprisingly difficult to get. It's also too big for many types of events that are held there. Vocal recitals, for example, almost never sell more than 400 to 500 seats; Jordan has just under 1000 seats, which means that the hall feels way too empty. A smaller hall would both be cheaper to rent and make for a more satisfying concert. (The disused but intact Steinert Hall was once one of the city's busiest halls for just this sort of smaller concert.) Recitals, new music, early music, and chamber music would all benefit from a medium-sized hall. The demand certainly exists: between late September and early May, almost no day passes without at least one or two concerts being held in a Boston space that was _not_ designed to be a concert hall.
As for opera, well, ablarc has a point there. But if you take all the local productions together---BLO, Opera Boston, Teatro Lirico d'Europa, the NEC Opera Workshop, BEMF every other year, the occasional production by H and H or Boston Baroque---then there is almost enough to justify building a proper house. One thing is certain: no existing space in town is particularly appropriate for opera. The Majestic is probably the best, but its pit is very small. Same for the Shubert, and its acoustics are not great. The Wang has a huge pit but the acoustics are downright awful. I haven't seen a production in the Opera House, so I can't say how it is.
In recent years, Opera Boston has shown that there is quite an eager audience for opera that isn't exactly standard issue (I think this year's run of Weill's _Rise and Fall of the City of Mahagonny_ sold out completely.) So it's also possible that a better venue would help the local opera scene improve.
Of course, if Constellation Center gets built, most of these problems will be solved, but that's still a question mark.
. Now that was culture!)