My impression is that the mentality has been shifted from "everything is core" to "almost nothing is core" (the toolbox idea). Given that, psionics has enough of a presence in D&D that it absolutely deserves to be presented with the initial playtest. This allows the skeptics to get comfortable with it, and the rules to be properly playtested in context. It won't suit everyone, but then again, neither will the classic magic system or any number of other rules.
Aeolius said:
5e needs to establish a framework for alternative "magics" into which psionics, incarnum, warlocks, witches, binders and the like can be grouped. The crunch should be similar, to allow for seamless integration into digital assistants, but the fluff would be different for all.
Frankly, I think the absolute opposite is true. Every different type of supernatural power should have completely different mechanical underpinnings. Otherwise, there is (as others have said) no real reason to call out these different types of powers.
Psionics ought to be a highly intuitive skill/feat/ability score based system with no slots, points, powers, or any other extraneous mechanics. It should feel completely natural, and thus completely different from the classic but hackneyed Vancian magic system.
Plane Sailing said:
Frankly I don't think they have a place in magical fantasy, it is a sci-fi trope. In a fantasy game they are just spell casters with a different backstory IMO.
I think they actually have more of a presence than most people realize.
For example one of the things 4e got right (hey, it happens) was putting monks under the psionic envelope. The whole eastern 'qi'/'ki' concept has something of a science-y feel to it and doesn't mesh with traditional D&D flavor, but is absolutely a valid fantasy trope and feels very 'psionic' to me. Similarly, I wrote a psionic class based on the idea of a yogi.
I think the seer is also more appropriate as a psionic archetype than a divine magic one (or, at least, both are independently valid).
Even given that it does have a bit of a sci-fi feel, so do D&D's Lovecraftian elements (including psionic monsters like illithids), or its Frankenstein-like flesh golems, or its steampunk elements (particularly Eberron). I certainly think psychic powers are more appropriate for fantasy then, say, warforged, or heroes with spiked hair and six packs or hourglass bodies. D&D accepts a wide variety of influences, and there's not a clear line between sci-fi and fantasy.