If they don't care about defense. You can run Grayson/Gordon with Book, and even KD doesn't play a lot of defense at this stage of his career. Nurk is the only one who will hustle on defense. So where does the defense come, if you score a lot? The moment you start to look at some defensive answers, you begin to lose your offense.
What teams are doing is rather simple. They double Book and KD. Knowing "the others" won't make enough shots. Of course if we run Gordon/Grayson all the time with KD/Book, the "others" will make doubles pay. But then, there's defense. With the lack of size between Grayson/Gordon/Book to defend bigger wings at the 3/2.
A lineup of Book, KD, EG, GA and Nurk can be at least a mediocre defense (Suns are currently 18th on that end). The problem is that this team should be a good offensive team and yet they are 14th on that end. They could get away with mediocre defense if they have a top tier offense (which they should), but they have struggled mightily on that end as well.
Having Beal back should help the offense and if they can find their way to around a top 5 offense and tighten the defense up even slightly they should at least look good in the regular season. How that translates to the playoffs is a whole other issue and we might need a trade to have a chance there.
Either way I think this roster of players is capable of much better basketball than we have seen so far. Simply having better defensive communication and rotations would improve the defense quite a bit. Playing faster with more freedom on offense while cleaning up some silly turnovers and this offense could be quite good. This roster may not be built to win a title, but they should still be a lot better than they have been.