Experiences in writing and distributing free software for a fee?

The FSF has long said that the "right to sell software" is an essential freedom, and "Distributing free software is an opportunity to raise funds for development. Don't waste it!"

I am thinking of going that route. That is, I would "sell"/charge a fee for access to my software under the BSD/MIT license; those who want it for no cost will be under a non-free license. However, I know of precious few companies which have done that. Most free software providers also make the source code available at no cost.

Can people point to examples of companies - preferably small companies which write their own software - which provide free software for a fee? (Other than RedHat.) What are the pros and cons? Is there much negative reaction to the idea?

My clients are pharmaceutical and biotech companies, who have no interest in redistributing any software they acquire, so I'm not really concerned about others immediately picking it up and reselling it, or incorporating it into other (non-free) products, or giving it away for free; to say nothing of providing support.

There aren't that many people in computational chemistry, and so it's rare to get code contributions from anyone else, so I'm not concerned about excluding possible involvement from others. While on the other hand, companies have money to pay for very expensive software.

Overall it seems like distributing free software for a fee is a great way to get money to fund a project - why don't more people do it?