You already answered your own question. It's a game mechanic explicitly included by the developers to allow players to mediate difficulty on their own. If the fight is too hard for you to solo, then the developers included phantoms just for you. The existence of NPC phantoms settles this matter pretty definitively.
Where's my "Accept This Moderation with Extreme Reluctance and Resentful Glares" button? When reading and responding to my posts, try to take it easy.
IMO NG is balanced for playing with the NPCs help. Except in the Depths Dragon fight where the NPCs are probably intended to fall to the dragon to make it appear more menacing.
I've been playing games like these my entire life (decades) and the bosses are just way way too difficult without summoning the NPCs. They are way too easy with the NPCs, but then so are 95% of games released nowadays.
Of course to not use the NPCs is missing out. And From' followed the same pattern with Armored Core where mercenary partners were gradually introduced as a game mechanic.
The real problem is there is no penalty for being hollow. So a player may well decide to just be hollow before getting to the first summon boss and never realize the NPCs are there to be summoned because it really isn't obvious that they would be.
EDITED: I thought the Anor Lando (O&S) boss was quite nice for that point in the game even though the design was pretty lame. It's for sure setup to encourage you to kill the fatty last as an out. Otherwise it isn't exceptionally difficult. Solaire had 90% of his health when I beat it, though I lost a couple times where he died first.
Remember, this is Gamefaqs: http://www.gamefaqs.com/boards/661226-pokemon-black-version-2/64668593