I'll try to boil down my typical long winded answer to 3 main points.

First, instruction is almost always free, so that's not a good reason to avoid one.

Second, the main difference between a sim and a real plane is that when a real plane crashes, you have to fix or replace it. Eventually, you will crash, however, an instructor greatly increases the length of time before the first crash. Additionally, you could hurt someone else, or something.

And finally, if you want to get first hand experience repairing planes, try flying without an instructor.

If you want to read more about learning to fly glow planes without an instructor, read some of the many threads that have already appeared about this subject.