The problem with this is that if they make Solo grinding the most effective way of progressing, then no one would party. Everyone wants to advance as quick as they can, within the time they have available to play. Make soloing the fastest way of leveling, and everyone who enjoys partying will get that taken away from them.

If partying (doesn't matter how; grind parties, dungeons, besieged type events, etc, etc) are the prime way of leveling, while solo content is STILL EFFECTIVE, then both sides win. People will want to party, and soloing will still be an effective means of progressing as well. This is how it should be.