Pressing TAB will suggest and import the player's coordinates in the delta argument of /particle. Since delta is a size, not a point in space, this isn't ever helpful.
As of 17w50a, it can also suggest ~ ~ ~, which does in fact input the user's coordinates in for the delta argument. Again, this leads to some incredibly unintuitive behavior, with the range growing based on your distance from the origin. I would expect this to be invalid syntax. ^ ^ ^ is considered valid as well.
Armchair analysis: this is probably because both the coordinate origin and the delta use vec3 brigadier argument types.
Just ran into this now. I think a better system would be to define a bounding box for the particles
/particle <particle name> <start location> <end location> [speed] [count] [force:normal] [viewers]
The main difference would be that this would allow you to use relative positions for the box, instead of just constants as the current system uses.