Mycorrhizae are more in evidence in highly organic soils than elsewhere. Look in a compost pile and compare it to any other media. Statements that trees use/encourage/support it in poor soils seems incongruous in that there is less food and or other amenable conditions available in poor soils. Yes, there is a symbiotic relationship, but is that relationship caused by the fruits of the union, or just coincident convienence?
You can overwater any media, and any media can get too dry. As long as your watering habits match the needs of your tree/pot/media combination, then it doesn't matter how retentive or draining it is. If you have Pines and Bald Cypress and treat them the same, one will die.