Even if it's "original inventor and heirs", you've created a liquidity process -- if you're a chemist with lung cancer, it would be great if you could sell your novel synthesis in order to pay for your cancer treatments rather than needing to start selling drugs yourself.
And then there's the problem of defining what a "practicing" entity is, of course. Someone who has at least one customer? Someone who has used the invention themselves? Someone who was planning on building a product, but didn't get into YC and ended up working at Microsoft instead while he tries to license his invention to raise money to pay for his next startup idea?