It is needless here to start bragging about the power of 5.56 and 9mm. You cannot always think in firepower as only front-line troops might have the luck to have only friendlies (aka fellow soldiers) and enemy on the field. Rear line troops are in the areas where civilians also happen to live and these small bores you so hate offer the lack of overpenetration ie minimal risk to non-combatants. Does any 5.56 and 9mm REGULAR ammo (standard military issue) offer the ability to not to shoot the granny with grandchild walking in the area behind the BG? 5.56 and 9mm (regular ball) do not lose very much energy in human body, up to 30% if lucky. So the bullet travels alot after going through the BG. Also the SWAT teams also mostly work in very habitated areas AND no harm must ever come to the innocents. Can your loved 5.56 and 9mm offer that? I do not count doctored rounds here and don't come to insult me with civ-ammo that military doesn't issue regularly. And 5.7x28 and 4.6x30 are originally standard issue military rounds so let's compare these to the appropriate ones.
PDW concept was conceived for REAR-line troops that cannot wield any weapon of considerable size, here are troops like medics, drivers, tankers, aircraft crews (Belgium Air Force issues FiveseveN DAO pistols, for example), guards and suchs; they cannot have anything dangle on them and distract from main job. Small caliber means low recoil (considerably smaller than 9mm and 5.56) which means accurate shots, whether in single or full auto fire, fast follow-ups (can shoot the BG quickly and with several bullets), also it means very stable auto-fire (P90 is very controllable and accurate in FA due small recoil) and list goes on. Yes you can argue that commandos can do the same with M4 and such but please try to remember as those beforementioned rear line troops are not any of that and may not be born-shooters. So the weapon must be easily handled and used by them. In ambush, like, the driver should be able to fire the gun off-hand and instinctivly as there might not be time so wield it properly. Can any regular shoot the M4 single handed from hip and hit the BG repeatedly?
Bear in mind that these guns the FN and HK offers are not competing rifle rounds, these are to fill the niche of small guns with rifle-ballistics and yet minimal danger to non-combatants and are mostly ment to be used in civilized areas where normal and ordinary people do happen to live. This gun-machismo is sometimes very ignorant ant wise should recognize it.