Well, my opinion is that they write these things to sell more only, properties and claims about baking soda are so inflated than the properties it actually does have, it is a very mild alkali substance, it does have mild cleaning properties and also mild water softener properties, considering that in powder detergents usually is already included there is no need to add further more, it would not be useful to anything....
Properites: could help a little in some cases to to bring your laundry whiter, does have odour absorber properties also as said very mild cleaning properties and water softener properties....
It is true it could allow you to use little less detergent further (as said most powders already include it among ingredients), at least depends from detergents you use and type, this speech indeed may be a little more valid for soaps and some liquid detergents.. soap is to be intended as plain soap...but absolutely not true you can use half amount of detergent or soap like I've read in some BS boxes or from producers...
It is not completely unuseful, it is just a complement that as said is already added in detergents .... for example added to " plain soap " can boost a little the power of the soap and as said allow you to use some less....
Just try to add same quantity of warm water in 2 bowls add in one some baking soda, then compare the quantity of soap you need for each bowl and you'll notice that to get water sudsy you will need less soap for the bowl in which you added baking soda...
Personal merit....well:
I used to add soda sometimes to some marseille liquid soap in laundry and it helped to "boost" it's cleaning properties and to use a little less... matter of 1/4 less to get same results....
So I think it depends from detergent to detergent and more than else the type of detergent you use.....
IMHO Borax is way more useful and effective to add in laundry than the baking soda, it is more expensive of course, anyway I mostly and usually don't use anything more than powder detergent so never have the need to add anything...