That's actually not exactly true. While it may not count towards PP or damage, not that I've cared to checked, it still reduces the boss's HP for everyone, and extremely fast with more people. Meaning less chance at dealing damage for others as well. As long as you're still consistently attacking it and joined early enough, you can not only finish the fight extremely fast, you can still easily net MVP spots.
Arthur by far has the best single target burst damage, as in being able to do the most damage in a short amount of time. Being able to charge her entire burst gauge on first turn and with her party wide attack up skill, not only deals a good amount of damage but increases everyone else's burst gauge after using her. Her 3~4x skill for additional damage and if you have another strong skill, you can even toss that on her for even more damage. Oh, and if you have Burst Time on from union, well then, lols gg. Great for battles in general and joining in late on raid bosses with a good chance at still netting MVP spots. Her only major flaws are her lack of debuffs and her inconsistency to time up her burst with the rest because of using her skill and boosting everyone else's, in which you could simply just use the skill again if your okay with it or just wait. So definitely better than Solomon with both costing 300 soul p.