you can use one if you like, many don't simply because they aren't needed, if they're using an airstone already. its ok to have one if you like, they add more space that needs cleaning too. any space in your pico reef that catches and holds waste particles needs to be flushed out occasionally
search out what this person does with a 1.5 gallon tank, to get it out to 5 years old now with professional level corals inside: Maritza the vase reef. if you use a square tank for your pico reef vs a vase, expect lots of work on topoff. if you use a vase, no work regarding topoff. at 1.5 gallons, a vase is orders more stable than a standard square open top tank, i must recommend vase only at that size for a new keeper.
MVR has all the details, check out one of their threads and study it and let us know what you think applied to the one you are considering.
a square tank is 100x harder to keep going safely vs a vase, regarding salinity control. the reason is due to how lids fit on a vase vs a square tank...evaporation control differences.