I think some people are misunderstanding what the Set Current Speed option is for. It's an option that many of us have been asking for. In almost all other cars, and the model S and X, when you engage cruise control the speed is set to the speed you are going. No matter what the posted speed limit is. This setting returns that simple functionality to the model 3 and Y for those who want it. So one doesn't have to screw around with offsets or whatever.
For instance, where I live, there are several school zones that are pretty long, and the cops can be overly zealous about enforcing them sometimes. The car thinks it's a 45 mph street, it doesn't read the school zone 15 mph speed limit signs. It's just easier to slow down, do a quick set into TACC and stay at 15 mph than worry about it.
It has nothing to do with anything that happens after you engage TACC/AP, it only affects (or rather, doesn't affect) the speed you are going when you engage it.
The school zones are indeed the biggest thing for me as to why I needed this. I like to set TACC to 20 MPH in the school zone so that the cops don't have reason for a pull-over, but was nearly impossible to do previously. Click the lever, speed gets set to 40 MPH, then have to emergency spin the thumbwheel downwards to drop the set speed quick enough so that the car doesn't speed up too much. So this setting is an extremely welcome addition.
But get this: I discovered you can get the best of both worlds this morning.
1. Put the setting on "Speed Limit".
2. Choose your speed limit offset (I have mine set to Fixed, +5 MPH).
3. Now change the setting to "Current Speed".
Now, when driving, you can actually select what the behavior will be.
1. If you want to set to current speed, just push downward on the drive lever briefly. TACC will set the speed limit to the current speed.
2. If you want to set to your speed limit + offset, push downward on the drive lever to the first detent and hold for 1 second, then release. TACC will set the speed to speed limit + offset.
3. In each case, follow up with a downward double-click to engage Autosteer.
So you can get the best of both worlds and choose what TACC will do.