Some people call Cleric/bards selfish so they don't have to consider how strong the classes really are.
It is mostly strawman and diversions.
The logic behind the restriction against using non-self-only buffs (in the case of clerics - I don't even know where bards would come into this) is that, if your options are a) apply a bunch of buffs to yourself to make yourself equal to the fighter or b) apply a bunch of buffs to the fighter, who is already as deadly as you would be with the buffs, the logical course of action is to go for b), because that gets the bigger bang for the buck. This is particularly true since the fighter can go off and be fighting people while you are buffing him (making those buffs useful immediately), while if you are buffing yourself, you're just standing there while the fight goes on around you.
It's not about selfishness in general, it's about selfishness that leads to bad tactics.
As for what that means with respect to the magus, I'm not sure. The biggest distinction that needs to be drawn is that the magus breaks the action economy argument once spell combat becomes a reliable choice at 8th level; they can buff themselves and make use of the buffs immediately. However, the magus spell list is actually pretty weak on single-target offensive buffs anyway, particularly ones that can be cast on other people -
bull's strength and
magic weapon (and
greater version of same) are about it, and the arcane weapon ability really makes
magic weapon and
gmw obsolete besides. So I guess it's up to you whether you would think that a magus should be "allowed" to cast
bull's strength on themselves or not.
(EDIT: for the record, the magus isn't likely to ever cast
enlarge person on
anyone except outside of combat, simply because it can't be used with spell combat. So I'm ignoring it - they can cast it on whoever they feel like before a fight, but I doubt most magi will even bother prepping it.)