You can access multiplayer anytime (provided that your game has been installed completely), even when you’re just early in the campaign, as long as you have a PS Plus subscription. Some parts are exclusively available in multiplayer mode only as well. Access to these parts are unlocked only after finishing your first multiplayer match. The only disadvantage for playing multiplayer early is your limited aircraft and upgrade part selection. Nevertheless, you’ll earn MRP by completing a match regardless of the results. Winning and scoring more will always yield higher MRP. Matches are usually very short (around 5 minutes each match), making it ideal to farm MRP efficiently.