Tourism is absolutely good for Venice.
I have to explain that Venice is not the island (fish-shaped) that everyone in the world call Venice. Our city is bigger than the "fish" and includes mainland (Mestre and Marghera, M&M: a medium size Veneto city over 250.000 inhabitants, with a chemical site nationally relevant and a port and airport areas, internationally relevants…).
So tourism economy is very good for this European city: as economy and as image as well.
If you look at DATAR ranking of European cities in 2003 (see the Datar site on the web and also COSES website) you can appreciate that the position of Venice is due to tourism, culture, chemistry, shipyards, port and so on.
Day trippers are an important share of today tourism and leisure economy. Also Venice, as many day-destinations, supports an heavy impact from day trippers but this phenomenon is inherent leisure economy and mass consumption. So Venice, as other historical gems, has to manage this kind of demand, not to demonize it.
From the culture point of view I think that tourism gives Venice its global resonance and that mass culture is very different from elite culture in the past Centuries. So magnificent heritage that Venice gives to the world is available thanks to local-national care but also thanks to tourism fame and visitors money. Nowadays, in Venice it is possible to produce culture only because its built heritage has been preserved thanks to big amount of public and private money. Non material culture is strongly linked to material one, and visitor audience is absolutely necessary to develop and maintain living and performing culture, today.
Socially speaking, tourism has a well known impact on local community (as in the other cities and islands!). Venice has to find a balance between the economic sustainability of a very peculiar water-heritage-city and the contemporary leisure economy.