The occasional street fair or parade is not really too bad a thing, though I agree clean up issues can make them less enjoyable afterwards.
When I was younger, and we first starting having parades (back then, all were on 37th) I recall one of my eledery neighbors saying how 'certain groups' had messier parades than others. Really, no matter the mess, its a city sanitation crew that cleans up, so I never understand how if the Columbian parade was messier than the Ecuadorian parade, it was anyone's fault.
Grabnted there are some parades and fair that generate more garabage (such as the flyers epsecially when politicians march), but honestly I feel that the parades and fairs add to the feeling of community and are worth the street closings and such.
Of course we can all do our part by not leaving our liter about when attending them. Yes, there will still be a need for sanitation to clean up ... and the only one that really really needs a heavy duty cleanup afterwards is probably Halloween (silly string, spilled juice boxes, candy wrappers, plus the fact the sheer number of marchers is TRIPLE any other) ... but every wrapper we don't drop, every flyer (even Monserrate!) we don't drop, etc., does add up.