I see that you referenced a "her." If that was Marlys (lady in her early - mid 60's), she tends to have strong opinions. So the proverbial grain of salt might be in order.
That said, mulching is a tricky subject. After placing over a thousand cubic yards of mulch on my own property over the course of a decade (and creating a 6 inch+ layer of muck), I've come to the conclusion that it's good in moderation.
Pros:
- eliminates weeds
- conserves soil moisture
- adds carbon and increases soil's cation exchange capacity
- holds supplemental nutrients better (less fertilizer waste)
- some tree roots appreciate the mucky consistency that results after decomposition (eg, lychees)
- encourages worms and other beneficials
- pushes pH towards neutral
Cons:
- causes imbalances between K and Ca resulting in lower brix (can be ameliorated with copious amounts of gypsum)
- locks up some micronutrients (can be mitigated with heavy and consistent application of fertilizer)
- causes phosphorous to skyrocket (not a good thing)
- many trees dislike the mucky consistency that results after decomp (eg, mangoes)
- can add nitrogen, depending on the mulch used (bad for mangoes)
- resulting mucky top layer (which retains more water + nutrients when fertilizing) encourages shallow root growth
I have 2 different lots. On the first lot, I created a layer of muck several inches thick. On the second lot, I added an inch or less, so I've been able to compare the two.
If you are good at fertilizing, you can create a very rich top layer of soil via the use of mulch, since the top layer (decomposed mulch) retains nutrients extremely well (this is a bad thing if you don't fertilize -- it locks up the little bit of nutrient that's naturally present, resulting in micronutrient deficiencies). I've had to add literally tons of gypsum to overcome jelly seed and increase brix on the trees on the heavily mulched lot.