The only reason to use it over shroomish(or oddish for that matter, use it people!) is if your making a sand team in which synthesis wouldnt be in your moveset and giga drain coming off shroomish wont be healing anything, it only 4HKOs frillish. re generator is nice but shroomish has more bulk which is crucial, also that extra speed which lets it outspeed some walls, the extra SAtk on foongus is nice but if you want SAtk use oddish which has better stats across the board, only havign 25% less accuracy with its sleep move.
Why are so many people choosing the same pokemon, what happened to originality?
Not often you see rest in LC or even seedot eigh?