IMO the reason Gentry said having a set 2nd unit wasn't a good idea this season was because:
(a) he had to find minutes for everyone even though we had too many wings - and if he was going to drop anyone out of the rotation it would have logically had to have been either Hedo or Childress, who had both just been brought in with massive, long term deals in the offseason, so he most likely felt compelled to play them and hope for the best (time will tell how this improves now that Hedo is gone)
(b) as you said yourself, we lost Lou in the offseason so without him (and Barbs) to anchor the 2nd unit it was no where near the same as last season - he went away from sticking with the set unit because it proved inferior to last season's 2nd unit, and instead went with players based on matchups etc.
It's not like he has the same or similar unit this season and just went "screw this, let's just randomise everything and mess with the chemistry of the 2nd unit" as you seem to imply. I can see the reasoning behind it.
Its true that the FO saddled Gentry with a very ill-conceived conglomeration of players and I predicted in the summer that if anything were going to cause the demise of the second unit, it would be Alvin trying to fit them all in.
You cannot argue that the second unit this year was inferior to last year's because there has been no second unit this year - just a bunch of individual subs. It certainly didn't come as any surprise to me that used this way that they were much inferior.
Barbs was in no way an 'anchor' for the 2nd unit - losing him was a blessing. Childress is a far better defender and given the way Barbs played last year not a big drop off on offense - at approximately the same salary, contrary to what you claim.
Now, I'd never think that Gentry was intentionally messing with the chemistry of the 2nd unit - but its just as disheartening that he doesn't see the value of it. Losing Lou was a blow but he didn't even try to sustain it and build on it. Watching that unit come together and seeing them defending well was one of the most hopeful things I've ever seen on the Suns in forty-some years.
Its not like Gentry came up with some new plan for the team that improved on last years plan. He's been bumbling along with no plan at all - and its hard to argue that the team hasn't looked like that whatever one might imagine Alvin has been trying to do.
The obvious choice for the second unit at the beginning of the year would have been Dragic, Dudley, Childress, Frye and a replacement for Lou. The primary contenders for his spot would be Lawal and Clark - big enough to play PF, young enough and not proven to be poor defenders. Let them practice together and play together - and assign the best defensive asst coach to run them. (The problem this creates for the team as a whole is PF on the first unit but using some mix of Hedo and Warrick couldn't be any worse than what transpired.)
There are some benefits to the plan aside from having a worthwhile second unit. It is no longer tied to an offensive strategy designed around Nash - imagine that, preparing for the future. Nor is it tied to a defensive scheme that caters to covering for weak individual defenders. Why, its almost too good to be true!