Yeah, I mean anyone can build a simple website, but ask that anyone to build a simple search engine... or a shopping cart... or whatever else custom you want built. Sure you can use some pre-fab script set, but as soon as you need to customize it, or your company out-grows the current implementation, you're screwed.
If its a custom application, you have the freedom to edit and tweak and redo parts of it that arn't fitting to your specific application.
I personally know this from experience... I work for a company that sells custom printed clothign and embroidered clothing, custom patches, etc. Before I started, the company bought a pre-fab already built CGI shopping cart called Shopsite ( WORST PROGRAM EVER! ) and now we're tied into it. Problem is, about 50% of the application isn't relavent to us, and it makes EVERYTHING a NIGHTMARE to manage.
But we can't change the program itself to make it more fitting, and we can't abandon it now 'cause no-one wants to invest the time or money to build something custom. So instead we WASTE MONEY trying to make-due with what we have.
We went with something akin to Dreamweaver, and for a half-year it was great. Now we've out-grown it, it's impossible to manage, invested tons of money in it, wasted tons of time, and can't get away from it. Pre-fab scripts and Dreamweaver GUI are great when you're a mom-and-pop group whose website rarely changes and never grows... but once you get beyond that...
If we had gone ahead and spent the time to learn the logic and shopping cart concepts and built our own, we would save eons of time and valuable money... we have probably 2 full-time jobs right now which basically serve just to work around the system we bought that doesn't work as it should.
As far as that JavaScript... rollovers are really simple :-) Dreamweaver / Frontpage allow Javascript rollovers through simple pre-built scripts that do just that... rollovers :-) All they do when you tell them to make one is substitute image names into a pre-fab Javascript template.