I was going over some spell and just noticed something odd...
The spell Supressing the Wizard's Handiwork says: "This spell is designed to be held while you do something else, so the magus gets a +3 bonus to all Concentration rolls." Ths leads to the obvious question... if I can design Supressing the Wizard's Handiwork in this way, why couldn't (and wouldn't) I design ANY Concentration spell in the same way?
I think I'm about to have a house rule reducing the ease factors for sustaining Concentration spells by 3.
Apart from the fact that it is done in these two instances?
Both spells are of course pretty useless unless the caster can make a concentration roll. Patient Spell is a bit less open than +3 to all concentration rolls (it only gets an effective +3 bonus to the roll for casting another spell)
Both are vim spells, the form where you can cast two spells in the same round with an appropriate concentration roll.
I'd give the bonus if the spell necessitates it and it is a vim spell. Central rule and all, the guidelines are just that guidelines not rules.
As Erick says, I think the key is that the spells are useless without being able to concentrate is the basic justification. I would be prepared to let any spell with a similar use get a +3 Concentration bonus (so, many of the Vim guidelines, if invented as Concentration Duration would be good candidates). I wouldn't allow it for just any random Concentration Duration spell --- I wouldn't allow it for a Creo Ignem "nuke stuff" at Concentration Duration spell, for example.
A further wrinkle is whether the bonus applies to Spontaneous Casting. I would be tempted to say no, it only applies to Formulaic casting. This is because a Spontaneous spell hasn't been designed at all, let alone designed with a specific use in mind.
I wouldn't, because a Spontaneous spell is so designed on-the-fly that you can change R/T/D after rolling it. In fact, I would say the lower EF is because your Concentration is on the same Target as the spell. If you push that way, casting a second spell on a rock you are keeping up with Levitation of the Ominous Stone should be easier. And that doesn't seem bad at all.