The advantages of code reuse are obvious; large pieces of code need only be designed, coded, tested, and documented once. Whether it is a class constructor, a function, or an XSL style sheet, if at least part of a solution is already written, you're that much closer to delivery of the final application.
Another issue is that developers can be insulated from the ins and outs of the various web browsers. No longer is there a sharp learning curve ahead or the feeling of hopelessness associated with trying to make something work in Internet Explorer while trying not to break it in Firefox. I have to admit that at times I've fixed a web page in one browser only to find that in the other browser it was fixed in the same way that the vet fixed my cat, Moreta.
The important thing to remember is that if you can complete three web pages in the time that it takes for Igor to complete one, who do you think will be shown the door the next time that the layoff fairy pays a visit?
Unfortunately, some development shops still cling to the outmoded idea that the better programmer writes more lines of code. Thankfully, this idea is going the way of the three-martini lunch. Gin, yuck! When you get down to it, the biggest possible problem is that if one of the constructors has a bug, every page that uses that constructor either directly or indirectly has the same bug.