That's basically the same reasoning that applies to any server-side scripting language: high level scripting languages are much more efficient to develop with than more structured lower-level languages, and the web-application model lets you use HTML as your GUI which is quick, easy, and flexible (though not as rich or stable as client-side GUIs can be.)

My personal preference for server-side development is Modern Perl, because it lets you develop anywhere along the "highly structured" to "rough scripting" line that you please. Other server-side languages are fine if they suit your development style, but I like to adjust my style to suit the project, and to use a language that can adapt to whatever style I need.

Client-side, Javascript is great especially with jQuery. I couldn't deal with javascript development without selectors; it's far too much effort. I'll have to give Javascript a try for server-side development, but I don't think I'd want to do a large project in Javascript. My current application at work has nearly 7K lines of client-side Javascript, and it's already getting unwieldy.