I understand that silverlight has come a long way since its inception, but I have always struggled to find the business need for silverlight. What are the advantages and disadvantages of silverlight 4.0 vs modern html5 and javascript libraries like jQuery or ExtJS?
Should i use silverlight or a combination of html5 and javascript for my ui?
I think where Silverlight shines (sic) is in streaming. If you go to www.dnrtv.com you can see how they use silverlight to stream content which I think is a great use of the technology.
As for using it in my web apps as the UI? Nah, I don't think it's of any particular use.
I think it has application where you want to stream or not use flash. :)
but this is only my opinion.
This question is going to be nuked by the question nazis. But it's a valid question, if phrased a little argumentatively.
A plugin free web is of a little value in itself. Just about everybody has flash installed, and I suspect the same will one day be true of Silverlight. If the benefit to the user is there, they won't mind installing a 5MB plugin.
So what does Silverlight offer that HTML 5 and javascript do not? A development model that doesn't suck. Seriously. It gives you a stripped down version of the windows desktop programming model. Creating software is faster, easier, and cheaper for Silverlight than it is using open web technologies. The software loads faster, runs faster, looks slicker, and you can do things that you cannot with just a browser. It's so nice to just target one platform instead of the hodgepodge that is the browsers that make up the internet (although in recent years this has become considerably less painful, it's still a mess.) So as a developer who's done a lot in both Silverlight and HTML/css/jQuery, I'd say I'd rather see a Silverlight dominated web than the "open web" any day. No I don't trust Microsoft, but neither do I trust glacially slow standards committees. Silverlight is seeing a new release every 6-10 months. HTML 5 is still vaporware. This is my opinion based on personal experience and some anecdotal comparisons of desktop vs web application development costs.
One of the scenarios that Microsoft aimed Silverlight at was LOB (Line Of Business) type applications. In this area it seriously rocks - it is delivered via the web, and its performance in each of the supported browsers is identical, i don't need to code different versions for different browsers.
HTML5 is still an unratified and evolving standard, and different browsers each have different bits implemented. I have nothing against HTML5, but once it is final (if it ever gets there) you are going to get more of the same as what has already happened - the different browser makers all implement it slightly differently, consequently there will be differences between the browsers which you as the developer will have to make allowances for.
With the release of Silverlight 4, MS have made big advances, and it kicks ass even more, giving access to webcams, printers, running out of browser, accessing the file system if trusted, etc. This makes it even more desktop like - you will never achieve this with HTML 5 and jQuery, they remain a strictly browser/web based technology.
Silverlight has been undergoing frequent releases and has a large (and growing) uptake. What you should think of too is that Silverlight is starting to become quite a mature technology, whereas HTML5 hasn't even really been born yet.
I have seen a few presentations about Silverlight but that's about it, never done much with it. The one question that comes to mind when I try to answer yours is what benefits does it have over Flash ? If the benefits are few or nothing major, then I guess you could also ask why has flash rarely picked up on business applications and it'd probably be the same for Silverlight. Although I suspect creating Silverlight applications is probably easier to do than Flash, but that's just a guess... (seems more developer oriented than designer oriented)
In any case, I think that as with any questions of "What tools should I use ?" you need to keep an open-mind but also try to pick the rights tools for the right job balanced with the ones you are the most comfortable to work with. I'm getting really comfortable working with ExtJS and I hope to continue working with it as the more it goes the more productive I am.