In general, that's the nature of the beast when it comes to mechanics like these. No matter how they balance it, there will always be "one thing in the pool of potentials" that is less desirable than the others. The challenge is making it so all the "potentials" will still be functional and acceptable regardless of their optimal-ness level. In this case, being RR Fodder is still better than a person staring at the card and blanching, so I find it acceptable in my mind.
LOL, oh God, I don't think I could Theorycraft that out in an hour, it'd probably take me sometime to figure out ALL the permutations of that. I could try to think up something but don't hold your breath on it any time soon, lol.
Though if they buffed Spear like that, it would certainly make Spear significantly more desirable. I got a feeling actually programming that in though is much more difficult than us discussing it is.
[EDIT]
Some food for thought since Richard brought it up and made me think about. Let's discuss Spear over the duration of one minute:
Ninja can Ninjutsu 3 times in a minute for a total of 1,080 Potency in that time frame.
Assuming you draw Spear twice in a row and do it just as their Ninjutsu goes on CD, they'll get Ninjutsu's going off every 16s seconds. Effectively they'll get four Ninjutsu's in 64s, giving them a 360 Potency Increase in that about one minute time frame.
Ninja's most damaging combo is their Aeolian Edge combo, which is a total potency of (150 + 200 + 320) = 670 Potency. Assuming no haste, no poisons, no slashing debuff, no buffs, absolutely nothing - Ninja can perform this 670 Potency combo every 7.5s for a total of 5,360 potency. Add 10% from double Balance (or Extended Balance) and you get an additional potency of 536 which is much better than what double Spear can give your Ninja DPS. This also doesn't include the fact that Raiton itself will be affected by Balance as well and you'll up with an additional 108 Potency over those 60s.
Just a bit more food for thought in terms of probability:
Drawing two Spears in a row is a 2.77% chance of occurring (Extended Spear is not the same as double Spear due to how the spacing of Ninjutsu - spear would wear off after the third Ninjutsu and push the 4th Ninjutsu to 68s)
Drawing two Balances in a row is a 2.77% chance of occurring. Drawing Spear and RRing it for Extend then drawing Balance is also 2.77% chance (you'll probably end up using Arrow unless TP is tight at the time) - this effectively makes drawing double Balance / Extended balance a 5.54% occurrence.
Neither of these probabilities take into account the usage of Shuffle.