Did some more research and found you are right to an extent. I don't grow hydro myself but here is what I found:
For hydroponic systems Mycorrhizae does not do well in hydroponic systems as it is sensitive to the nutrients that are used for hydroponics. For hydroponic systems it is best to use Subculture or Hydroguard. Both these products contain bacillus strains which are well suited to hydroponics even in systems that have high salt contents. Subculture should be applied at a rate of 1 teaspoon per 10 gallons of water/nutrient solution (1 pouch per 50 gallons). The first application of Hydroguard should be 15ml per gallon and maintenance applications at 5-10ml per gallon of nutrient solution. For coco fiber, rockwool and Ready-Gro mediums, both Hydroguard and Subculture should be reapplied 21 days after first application and thereafter every 21 days. For hydroponic systems that use Hydroton rocks, perlite, NFT or aeroponics, the Subculture and Hydroguard should be reapplied every 7 days.
The problem with this is that these products can quickly become very expensive/difficult to maintain. If you're not a hydro grower extraordinaire, it'll be a real challenge. If a hydro grower does manage to get a beneficial bacteria colony attached to the root system, the problem is that there isn't any Mycorrhizae, one of the MOST beneficial micro bacteria out there.
The naturalistic association of soil with bacteria provides the mycorrhizae with relatively constant and direct access to carbohydrates, such as glucose and sucrose supplied by the plant. The carbohydrates are trans-located from their source (usually leaves) to root tissue and on to fungal partners. In return, the plant gains the benefits of the mycorrhizae's higher absorptive capacity for water and mineral nutrients (due to comparatively large surface area of mycorrhizae:root ratio), thus improving the plant's mineral absorption capabilities immensely.
Plant roots alone may be incapable of taking up phosphate ions that are demineralized, for example, in soils with a basic pH. The mycorrhizal fungus can, however, access these phosphorus sources, and make them available to the plants they colonize.
Over all, growing hydro is a constant job that many people cannot dedicate all their time to. And that is what it takes, careful observation to make sure everything is running smoothly as things can turn on you very quickly with hydro. To me, it's too much of a time-consuming pain to deal with. Soil works just fine for me and it's a lot cheaper and easier to deal with IMO