I've seen several people mention this (and making the field smaller!) as a 'solution' to open up the game, increase scoring, etc. The off-sides rule is there to help open up the game.
It allows the defense to push well up to mid field, allowing better support for the attack; support (depth) being an attacking principle in soccer. This also allows for a better (faster and more successful) counter-attack.
If there were no off-sides, an attacker or two would cherry pick inside the penalty area and as a result, 2-3 defenders would also stay back there. No support for the attack, no counter-attack, less scoring, bad idea.