The issue is that by the time you bring in an ORM/database migrations, form builder, link builders, asset pipeline, admin panel, etc you have basically hobbled together your own ghetto version of Rails without the vast resources/support that Rails has. One of the benefits of Rails is that it has all of those features mostly baked in - at the cost of some bloat.

But my project doesn't need all that, you might say. Well, that's fine, then use something like Sinatra. But I wouldn't build a non-toy web application without the above things.