I strongly agree with PJ's comment. Plugins like Flash, Silverlight, and Active-X complicate the process and lower the quality of the web experience.
- Users must install and maintain an additional piece.
- Less likely to work in all environments (mobile phone, etc.)
- Plugins bypass the authentication mechanisms built into browsers. Maintaining session currency requires sharing information between browser and the plugin - extra work to develop and many Flash/Silverlight/etc. developers just don't bother.
- Plugins may provide an additional vector for malware. They may not be harder to secure than native browser stuff but you have to worry about two things. E.g. Flash cross-site scripting attacks.
- Requires more than one set of development tools (e.g. Firebug can't see inside Flash or Silverlight objects on a page)
- Unnecessary most of the time: E.g., a lot of common Flash animation can be done equally well with native browser techniques like Javascript, CSS...
- Future browser technologies will make plugins even less necessary: e.g. Flash was probably the best option when YouTube came out but when HTML 5 starts to take over the video and audio tags may make it superfluous.
If your web app requires some plugin you WILL lose at least a small part of your target audience - Silverlight more than Flash. If you don't absolutely have to have the functionality a plugin gives you, why short-change yourself like that?