Most web businesses of today believe web applications that require client plugin will fail.  This is only partially true.  First, people have no problem with installing useful applications, so an application can install the client.  Second, there are ways to grind away most biases against client plugins.

One way is best described with a story about an old women who believes her son is the most peaceful man in the world.  One day a friend of her tells her that her son just killed a man.  She laughs her friend off, disbelieving the news.  Then a close friend of her son runs over and gives her the same news.  She tells him that can't be true because her son is kind and gentle, unable to kill a man.  But a seed of doubt has been planted and she feels nervous.  When she overhears her son's name mentioned by two complete strangers talking about a murder, she starts wailing and rushes out looking for her son.

A plugin useful to many web applications will be installed by the users.  Flash, Acrobat Reader, and Adobe SVG plugins do get installed because they are generic and useful to many.