Balance and diversity

The first consideration is forms with very high hit point regen.  These are necro Horrific Beast (HB, which is 18 HPR) and ninjitsu Dog and Cat forms (16-30 HPR, depending on Ninjitsu skill).  I assume the devs feel it is unbalanced to let these forms do spells and special moves in addition to their HPR.  Note that HB can cast and use special moves if your necro has a "focused" build.  That means no other spell book skills greater than about 30, nor parry, nor taming, nor music skills.  Cat and Dog forms can never do the special stuff except for Shadow Jump and Mirror Image ("clone").  Also note that in their various ninja forms, shurikan and darts are also available, giving you a ranged, and possibly poison, attack.

That leaves the remaining Ninjitsu forms, where I don't see the answer as so clear cut.  Again, they are all allowed Shadow Jump and Mirror Image.  The former is not very useful in battle, but it can be very useful in traveling between battles.  I didn't understand the latter until recently.  It is actually a very powerful battle capability, regardless of one's form.  Think of it as a low cost (10 mana, reduced by any lower mana cost properties), one-use 100% shield.  Each counts as 1 pet, so you can have multiple copies at once.  A foe attacking you has a good chance of hitting (and destorying) a clone instead.  So even though you have no way of healing, overall incoming damage to you is greatly reduced.  Combined with the 30 HPR as dog or cat, your battle capatility is quite high.  The other forms except rabbit/rat (greater stealth) and llama/ostard (be your own mount) all offer special battle capabilities.  There are actually four other forms which are available via Talismans available via Heritage Tokens.  Two have additional battle capabilities.  The Cu Sidhe form will heal itself (you) if you have bandages.  Don't expect major heals, but combined with you spamming clones it is quite effective.  I haven't used Reptalon, but it offers a fire breath on top of what you are doing with your weapon.

BTW, an elf can wear woodland armor, which when made of heartwood offers 2 HPR per piece.  Thus a full set is 12 additional HPR on top of what HB or cat/dog offer.  Exceeding the cap is not an issue, because the cap is only for wearables, not skills and racial traits.  A greater dragon will still be a major problem, but it will often smash a clone instead of you.  Lesser foes have a hard time doing sustained damage due to the combo of you out-healing their damage and misdirecting their attacks onto clones.  Add well-timed smoke bombs to quickly hide, and you have exceptional hit, run, and re-hit capability.

@Urge I believe the only necro form with major limitations is Horrific Beast in non-focused builds.  If the build is focused, HB can do everything but ride a mount.  Regardless of focused or not, Wrath and Lich forms have always been able to cast and use special moves.  I'm sure Wraths cannot ride mounts, but I don't know about Lichs.

Regarding samps, I agree with you that they are OP, and I would not want all other builds to be overbuffed to compensate.  People often suggest that using Chivalry with Necro forms should either not be allowed or cause penalties to the Chiv abilities like Enemy of One.  That would be okay, I guess.  But another way to accomplish it would be to make the various form buffs of the builds dependent on Spirit Speak, essentially requiring that significant SS be added to the Samp build to make HP leach (vamp), mana leach (lich), stamina leach (wraith), and HPR (HB) effective.  In an already tight build, this would force the player to choose what he needed to emphasize.  Basically, the higher the SS the more effective the buffs.  At 100 SS, the character would have full effectiveness as they have now.  But to give something back after taking something away, the buffs would continue to become more effective as SS skill approached 120.  This effect would be based on actual SS skill, not jewelry or JoaT.