Sort of a tangent here but I've been trying to decide if our organization needs to move away from using Drupal for custom application development. You see to have some experience, what are your thoughts on it for highly customized web applications?
I said this to my wife this morning - "you know what, building stuff in Rails just feels like it accesses my creative brain more than anything I've done before." Having been exposed to some of what Acquia is doing with their cloud hosting stuff I can say with a fair amount of certainty that almost anything under the sun can be accomplished with Drupal, but that's (the creative bit) probably why I'm on a Rails kick lately. It's just more fun (to me). The overhead does feel lower, too.
Sorry if that didn't lend any insight to your actual question.
Drupal is kind of a framework and a CMS in one, it's somewhat unique. It also seems that in PHP land a lot of the frameworks are converging on a standard, it seems like Symphony 2 will be used as a base for other projects (like Drupal 8). The point is, Druapl gives you a lot more out of the gate than any other framework in PHP or any other language.
The "con" would be that you have to work in PHP and work with their relatively straightforward imperative API instead of cooler OO APIs with ORM (Though Drupal 7's revamped DB API offers some new interesting options). If those barely register as cons with you then the only reason to change and abandon already built infrastructure would be something those languages have PHP/Druapl doesn't like some binding for some cool feature.
Ah, one other con for Drupal, it does have a lot of over head and can at times be slow. But then again, easily if not more so can Rails have overhead and be slow. They like to brag about their > 1000 lines of pluralization code, others cringe. Smaller frameworks might be faster. Django possibly, but I really have no idea.
So it comes down to what you are doing and the requirements. I want to learn them all so I can better evaluate them all and in the future more knowledgeably pick the best tool for each new job