All manufacturers of acrylic color recommend that pure acrylic medium/gel is not to be used as varnish. Is this due to the adhesive nature of the acrylic medium?
When you use an acrylic medium as the final coating, you may get an even sheen and the look you are after, but acrylic mediums are just another layer of acrylic paint without the pigment. Yes, acrylics are very durable and will hold up over time, but the main problems are 1) They are porous. Dirt and dust attach to the surface, and when moisture from the environment begin to collect on the surface as well, the grime begins to penetrate into the acrylic layer. Over time, this contaminates the painting, and at some point a conservator will have to figure out how to remove this grime. 2) They are non-removable. A true varnish should be removable, mostly for the above reason that grime needs to be removed at some point in the painting's life. If the grime is in the varnish layer, it can be stripped off fairly easily and quickly, and the painting is restored back to its original look. In the short term, removability means if there is a problem during application, or shipping, or just some sort of accident that damages the surface, you can remove it without disturbing the painting itself.

Now, does this mean that EVERY painting needs to be varnished? Well, not necessarily. Although there are the obvious benefits, the problem is that these coats are generally applied as a full coating, and some artists use the varying sheen of the layers as a crucial part of their artwork. For example, if you have a painting that has Black Gesso as the background, and the splashes of mediums and color over it that are high gloss, if you were to varnish it, you would lose the variance of sheen and this changes the artwork.

In cases like this, I think it's ultimately up to the artist. They may feel the varnish is critical and coat the painting with gloss and "in paint" the matte areas with some matte or satin varnish. Most of the time this is near impossible and the artist chooses not to varnish at all. When this is the case, it's important to inform the buyer/ gallery/etc. so that they can address it with frequent dusting and cleaning, and/or choosing an environment of lower humidity and constant temperature.