Ozone isn't what I would call a gimmick. It is a valuable player in certain kinds of pools. Ozone is especially useful in spas that use chlorine and in outdoor commercial pools. Ozone is oxidizer but not a sanitizer. Chlorine is both an oxidizer and a sanitizer. In commercial pools and spas of all kinds there can be a lot of CC that accumulates, more than chlorine alone is able to deal with. In those situations, ozone (or UV) is a very handy addition to help keep the CC level under control.
You have to have a sanitizer. Assuming you use chlorine as your sanitizer, it is also an oxidizer. In an outdoor residential pool, things that need oxidizing, mostly CC, are fairly rare and normal sanitizing levels of chlorine, plus sunlight, have no problem keeping up with the required oxidizing. Thus, there is little for the ozone to do.
The only normal exception in a residential pool would be a major pool party. Even then, the benefit is minimal, since a typical residential pool normally has several days to recover from a pool party, and chlorine will catch up in a couple of days. Unless you frequently throw major pool parties on consecutive days, there really isn't much of anything for the ozone to do.
Spas and commercial pools are different because they have many people swimming compared to the amount of water they hold. Because of that, CC levels (and levels of other things that need oxidizing) are high and sanitizing levels of chlorine frequently can not keep up with the oxidizing load. In those situations, having something like ozone (or UV) can be very handy.
Ozone normally breaks down very quickly, and a proper install is designed for it to break down before it gets into the open pool water. If it survives into the open pool water it can outgas and cause respiratory problems. Because of this, ozone only acts on the water as it is passing through the equipment pad area (and typically some of the piping on the way back to the pool). That means ozone has little effect on things like algae, which grow in the bulk pool water and reinfect the water returning from the pump fairly quickly.