Main lines = Better results

Many players choose sidelines (including gambits and systems like the Hippo) because they seem to be more practical. However, this isn't necessarily true. As an experiment, I played some blitz games with sidelines I had studied in depth and then played some games with main lines I had not studied.

My results were about the same. I believe this was because the main lines led to better positions which compensated for my lack of knowledge. I can only imagine what would happen if I had studied those specific main line openings in greater depth, but this is encouraging if you lack study time. Also, playing main lines does not have to mean the Najdorf Poisoned Pawn.

Below is an example of what can happen if you play passive sidelines. It certainly didn't benefit my opponent here at all. And also, they had very little counterplay (Despite me not playing perfectly!).

In practical amateur games, there are so many players avoiding main lines that you might actually have a higher surprise factor by playing main lines instead of sidelines. For example, around half of club players avoid the Open Sicilian as white. Basically, the main lines have a higher surprise factor than sidelines at amateur level, with a better position added as a bonus! Below is a game where I played the "theoretical" 3. Nc3 against the French defense.

If you are playing opponents at a higher level, it feels safe to be able to rely on moves recommended by grandmasters. Often, stronger opponents will play inferior lines to try to win, which will give you an advantage against them. Or, they will play into your preparation, where the best they can achieve is a draw.

Finally, deeply learning main lines is the best way to play at a high level if you aren't a chess prodigy. It limits opponents' options and reduces the amount of thinking necessary during the game.