There are many reasons to buy a premium WordPress plugin but following are the most notable reasons:
Developers offering premium WordPress plugins tend to stand behind their plugins and provide good customer support. This is a rarity for free WordPress plugins on most cases. A free plugin developer can’t really stand behind his plugin and answer your questions if he is not getting paid in some way.
Better Plugin Codebase
Free WordPress plugins are sometimes developed by hobbyists who do not have a good grasp of WordPress’s coding guidelines. This can result in a poorly coded plugin with potential security holes.
Also, good premium plugins are usually developed by knowledgeable coders who keep the site performance in mind when developing the plugin. This ensures that your site’s performance won’t go down after you install the plugin.
Continued Development and Updates
Premium WordPress plugin developers usually continue to update their plugins’ coding to fix bugs, add features and improve the overall design to keep up with current web trends. You usually get these updates for free.
Premium WordPress plugin developers will also keep the plugin upto date with the latest release of WordPress. This ensures that you won’t get stuck with an incompatible plugin down the path and no chance of getting any help (when you have active customers and members, this is a situation you DO NOT want to get into).
Premium WordPress plugins usually offer more functionality and has superior back-end usability. This can give you an immediate advantage over your competitors.
If your competitor is using good premium plugins and you are only using the free alternatives (knowing that the premium version is better) then you need to acknowledge that your competitor has better tools than you do. It is hard to win against an opponent who has superior technology!