I personally want pH fluctuation. The charts show 5.8 to be ideal for hydro, but if held strictly to 5.8, all of the elements are still not available. I like that I can pull it down to 5.3ish and let it climb up to almost 7. This I feel gives my plant(s) more opportunity to uptake what it needs.
As far as PH perfect solutions, they likely use a lot of buffering agents in their formulas so when mixed with the likes of RO water, there is not much resistance in the water for the buffers to overpower and dictate what the PH will be once that struggle is settled.