O.K. Here's my opinion:
Flight simulator software has been available since "FS1" was published for the Apple ][ in about 1980. Despite being extremely crude with its wire graphics, it was a start (nobody had ever seen anything like it before) and it gave the average member of the (Apple) computer owning public some idea as to what flying an aeroplane could be like. A few years later "FS2 was published which was a "quantum leap" in graphics technology and gave a much better idea as to the visual cues to be seen from the cockpit of a light aircraft, although landing it was not easy!
In about 1985 a computer literate work colleague, who had absolutely no aviation experience at all approached me to ask about the art of flying. I gave him a disc (yes a 5.25 inch floppy, remember those?) of FS2 and gave him a rough idea as to what to do. He treated it as a computer game and spent many hours, with his computer keybard manoeuvering his aircraft and trying to avoid crashes. Eventually, he saved his position below a glideslope on the centerline of an ILS equipped runway (probably Meigs field with the tower on the left). Then he practiced trying to stay on the ILS all the way down to the runway. It took him a long time to manage to do this on his keyboard, but at least, when he came off, he could reset to his saved position and start again.
After several months practice, I offered him the opportunity to discover what he had learned by actually trying to fly a light A/C.
In a Cessna 152, he naturally had difficulty with the taxying and the R/T work, but after takeoff, it took him only a few minutes to discover that what he could see out of the real cockpit agreed with what he saw on his computer monitor, and he displayed a level of flying handling competance what I would not normally expect before about 5 hours of instruction.
Then we had a break, and I then put him in a fully IFR equipped Cherokee Warrior. After takeoff - with which he coped well this time, he had lttle difficulty in maintaining a desired altitude and tracking down to the local VOR. He then executed a procedure turn to return on a reciprocal heading. Shortly after this, I obtained radar vectors for him to the local "big" airfield's ILS. He followed his headings, established on both localiser and glidepath and then, with a mere 1 hour's genuine flying training under his belt, followed said ILS down to a decision height of 200' keeping both needles beautifully in the middle. A candidate for an initial I.R. test could not have done batter!
In the early 1990's I think, the IMC rating was upgraded. The training requirement was upped from 10 to 15 hours and test candidates had to demonstrate the ability to fly ILS's, NDB approaches and execute holds.
I found it very difficult to get the average candidate up to what I thought was a sufficient standard in the 15 hours, but when I memtioned to them that they needed to get a flight simulation program for their computers, the standard shot way, way up!
Of course since then flight simulation software has continued to improve and become more and more sophisticated. Apart from the uses mentioned above from the early days, pilots training on or converting to a specific aircraft type can use such software to help in these processes as so many of the operations of the systems in these modern aircraft are accurately simulated. Although, your home PC can never give the reality of an all singing, all dancing flight simulator, it is a very valuable adjunct to type conversion training, especially in the early "systems training" stage before use of the full flight simulator becomes necessary.
Please accept large doses of IMHO in the above post!
P.P.