Dedicated circuits

Yes, in theory your thinking is correct and as long as the installation is up to code (which it may be) in your area, you should be fine. Remember that local electrical code inspectors can vary in their understanding and interpretation of electrical codes, which can vary a bit by location.

The one issue for the discussion is that a 20-amp receptacle has a physically slightly different plug configuration which implies it is connected to a 20-amp circuit. Someone may be misled when they see a 20-amp receptacle thinking they can use a 20-amp rated device. Of course, the 15-amp breaker would trip if the 20-amp device were to overload it so there should be little to no risk of an overload causing a fire.

**Disclaimer-I am not a licensed electrician**

if a 15A breaker was in front of a circuit that was made of 12-2 Romex and 20A outlets, wouldn’t that be safer? I mean, the 15A breaker would trip way before the 12-2 or the 20A outlet got hot enough to catch fire, right?

No you can not put a 20A duplex receptacle outlet on a 15 amp branch circuit. The breaker handle rating determines the size of the branch circuit, not the wire. The wire could be #10 and if the breaker is a 15A the branch circuit rating is still 15 amp.

You can put two 15 amp receptacles or a combination of 15A and 20A receptacles outlets on a 20 amp circuit, (20 amp breaker). You can install one 15 amp duplex on a 20 amp circuit. (Duplex is two.)

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So far I haven't seen anyone question why the dedicated circuit made such a difference. So here is the answer: typical wiring for receptacles is several, often two or more rooms worth, on the same branch circuit. Aside from the smaller consequence of the longer cable run to reach the last receptacle on the branch circuit, the major consequence is the number of splices or receptacles that are in the path. This is a major consequence because the splices are often less than optimal, and where the branch circuit is wired through one receptacle, and on to other(s) it is usually with the backstab connections. A chain of many weak links is what you get, unless you properly pigtail wire each receptacle. 
 

With that myth debunked, let's look at another myth: isolated grounds in dedicated branch circuits. By definition, when you do a dedicated branch circuit run, the ground is isolated. So what do isolated ground receptacles exist for? The answer is in commercial branch circuits, the circuit has a common junction box that each receptacle is wired into, in a hub and spoke topology. This can cause undesirable effects to the shared ground, from eddy currents induced in each spoke. So, where we want a "clean" ground, devoid of such electrical noise, we run a dedicated ground, and use an isolated ground receptacle to keep the noisy branch ground separated separate from the equipment safety ground. 

The breaker handle rating determines the size of the branch circuit, not the wire.

@jea48 okay, I understand that. But what I was getting at was wouldn’t a circuit with Romex and outlets rated for 20A but protected by a breaker that will trip at 15A actually be safer? I mean the 15A breaker should now trip way before the 20A circuit can get hot enough to be dangerous?

@jea48  It's against building code to install 20A outlets on a 15A breaker.  You may be correct that the 15A breaker would trip, but do you want to risk your home insurance not paying out after your house burns down?  Just follow the code and be protected.  It's there for a resaon.