Webforms vs. MVC seems to be a hot topic right now. Everyone I know touts MVC to be the next great thing. From my slight dabblings in it, it seems ok, but no I don't think it will be the end of webforms.
My reasoning, and the reasoning as to why webforms would be chosen over MVC, has more to do with a business perspective rather than what one is better than the other.
Time/money are the greatest reasons why webforms would be chosen over MVC.
If most of your team knows webforms, and you don't have the time to get them up to speed on MVC, the code that will be produced may not be quality. Learning the basics of MVC then jumping in and doing that complex page that you need to do are very different things. The learning curve is high so you need to factor that into your budget.
If you have a large website written all in webforms, you might be more inclined to make any new pages in webforms so that you don't have two very different types of pages in your site.
I'm not saying it's an all or nothing approach here, but it does make your code harder to maintain if there is a split of both, especially if not everyone on the team is familiar with MVC.
My company recently did three test pages with MVC. We sat down and designed them out. One issue we ran into is that most of our screens have the View and Edit functionality on the same page. We ended up needing more than one form on the page. No biggy, except then we wouldn't use our masterpage. We had to revamp that so that both the webforms pages and MVC pages could use the same masterpage for common look and feel. Now we have an extra layer of nesting.
We needed to create a whole new folder structure for these pages so that it followed the proper MVC separation.
I felt there were too many files for 3 pages, but that is my personal opinion.
In my opinion, you would choose webforms over MVC if you don't have the time/money to invest in updating your site to use MVC. If you do a half arsed approach to this, it won't be any better than the webforms you have now. Worse, you could even be setting this technology up for failure in your company if it's messed up, as upper management might see it as something inferior to what they know.