The Freemium model is here to stay. People want to try your product before committing to paying for it. Especially advanced users who most likely buy into, at least some aspects of, the Open Source ethos.
WordPress plugins are no exception. This also means that your free version gives away are least one of your best features. The upsell should be tempting, but users have to fall in love with what they get for free.
Here’s my list of plugins I love and the free versions add a ton of value.