That is a valid point and something they are trying to address.
Orks are the 'free to play' race. You don't need to buy the game to play as one of the basic Ork classes but if you do buy the game (or just the Ork Faction) then you unlock the ability to spawn Hero Classes, vehicles etc.
The intent of this - and I remain sceptical as to whether it will actually work in practice - is that lots of people who want to play will join up for the fun but not stump up cash will play Orks. Further, Orks will have some kind of cumulative buff for the other greenskins nearby. So Orks should end up with the kind of Zergy mob style of fighting that they should have.
The big elephant sized fly in the ointment room is whether people will want to join in with this given that:
a) Looking at rough polls on the offical forum Space Marines are the biggest faction by far, then CSM, then a big gap before the Orks and finally the Eldar.
b) The Devs have stated that in a straight up fight a Marine can take on two or three Ork Boys...but a skillful Ork player could kill an average Marine player one-on-one.
*IF* there are many more Ork players than Marines then they might be able to put up a decent fight and things will balance out as per the TT game.
And I remember the massive faction swings between servers and factions that we had in WAR.
There is a lot of talk from the Founders about Eldar fighting the campaign very differently from the way SM and CSM do - that they will not be trying to seize and hold ground, for example.
I hope they get this to work but unless they can address the inherent population balance I fear this game may be SM vs Tyranid Bots a lot of the time.