I feel like in the modern NBA, given the amount of pick-and-roll, you need a ball handler/driver, a rim running big, and shooters around them, and it's up to your ball handler to make the right decision in the drive-and-kick game. Simmons obviously is in that situation most of the time, so whilst a 3 point shot would be handy, it isn't necessarily essential, particularly when you is so gifted athletically.
The Sixers tend to run into problems when they use someone else as the ball handler (eg. with Butler last season), or when they make a conscious effort to post Embiid in set plays. This basically takes away all Simmons strengths, and consigns him to the "dunkers spot". Part of the problem is on Brett Brown. You could use Simmons as your screen setter/roll man, but they rarely seem to do that (probably because that tends to put Embiid on the perimeter where he isn't truly effective).
So whilst the 3 point shot isn't necessarily essential (just look at Giannis and his hit-and-miss 3 pt shooting), it wouldn't hurt that he was at least partially competent, especially as he ages and his athleticism declines.