The better question to start with, is "What is an IDX"?
Your MLS software is powerful and useful to agents in your area looking to share information about available properties, showing conditions and more. MLS software providers over the years have standardized a method for exporting data from the databases these products maintain that allows for "syndication" or, copying to other external sources for public display and consumption.
Some MLS boards charge a "pass through fee" for use of this exportable database. Others allow use of the feed contents at no charge. This is known as a "Free IDX".
Important distinctions, and understanding how content is delivered on the page
An IDX, on it's own is just a small portion of the software, infrastructure and support needed to display listings on your site. A small flow chart would look something like this:
MLS software > IDX feed > IDX provider database > delivery to a structured html/css/js page on a website.
In all cases, the IDX feed itself is a database only, with no formatting, delivery or display capabilities. It's the equivalent of saying "The engine is free, so long as you put it and the rest of the car together".
What an IDX provider does:
An IDX provider is something like a car builder. If the engine is free, and the parts need assembled... the accessory performance parts, drivetrain, transmission, interior, and exterior are all customized by the manufacturer, and each company will deliver a completely different result. Some make Ferrari's, some make Honda's, and each of them is priced based on the various tooling, performance and expense that went into bringing the product to your doorstep.
The same is true for the IDX feed. An IDX provider will take the bare database and begin making a server and delivery infrastructure to be able to fetch changes, and deliver those changes to another destination. In the process, they will send as part of the delivery, a layout, functionality and user experience to go with the data.
For an IDX product (from an IDX plugin or vendor) to work on your website, the website's coding architecture must be compatible with the coding the IDX vendor used to build the service.
For example, if I had a v12 engine, I cannot fit it into a Smart Car without completely rebuilding the Smart Car from scratch. These two products were simply never intended to ever work together with each other.
Likewise, if I had a full size tractor trailor, I would not be able to put in a smart car engine without significant effort, and the results would be less than ideal.
Pairing an IDX vendor platform with a compatible website is an important step when considering what overall experience you want to deliver to end users.
What this means:
When your MLS provider offers "FREE IDX" what they typically are offering is free access to the database only. In some cases, this can also mean free access to a limited version of the MLS itself, complete with layout and functionality, but it's important to note that in most cases, this is not natively going to be compatible with the website you want to display it on. Typical inclusion is via IFRAME, and is not mobile scalable, limited in it's lead routing capabilities and in general is not a good end user experience. It is also impossible to build any additional communication, routing, or features as part of your website experience that work with the product.
We use WordPress IDX vendors, because they are built to work with our infrastructure
We believe in the power of WordPress, and in partnering with reputable companies who's specialty is delivering to you the best possible IDX experience for your end users. This leaves us free to focus on lead generation, website professionalism and design, and overall site performance.
We currently integrate with the top providers of WordPress IDX in the country, and believe you benefit from both a cost savings as well as an increase in total quality by being able to select from multiple product vendors competing in this space to provide you with the right solution that meets your needs.