This is a catch 22 situation at times
Let me start by saying that I absolutely agree with Pippin on this issue. We followed this line of thought when we added the CSS for Ninja Forms. The idea is that the theme should control how your forms look not a plugin. So we handled just a couple items pertaining to label positioning and a generic style to the datapicker.
Here’s the catch. People with well designed themes or a basic understanding of CSS love the decision. People with poorly designed themes and/or no understanding of CSS hate the decision and are very vocal about it. We found a compromise for us. We created a Layout and Styles Extension to help those who need it.
We’re sticking to our guns on this one as I’ve had to deal with plugins with excessive CSS far too many times. In most cases it’s a lot easier to add CSS then it is to override or take it away.
Checkout Pippin Williamson’s post, Write Simple CSS in Plugins for his take on this issue and some great tips.