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Is it necessary to say RESUME OWN NAVIGATION after every little route deviation?

Yes.
You either follow the flight plan route or the route given by atc, or a radar heading.
If we don't tell you resume your own navigation (and it should be qualified by the phrase "resume own navigation to 'xxx' <reporting point>), then you'd have to continue as per the clearance you were proceeding on.

(If that makes any sense).

Yet more crap phraseology. In these days of IRS/GPS and the instruction is Track Direct To ... , who do you think is going to do that navigation besides the pilot (or autopilot/FMS)

What do I mean? There's a route A-B-C-D. I ask an a/c to continue HDG A-B to avoid another traffic. Just a few miles, so there' not a single chance for pilots to get lost in modern glass cockpit. Then instead of saying 'RESUME OWN NAVIGATION TO -C-' I simply say 'PROCEED (LEFT, RIGHT, DIRECT) TO -C-', and the a/c then follows the route to -C-D. Any ambiguity?

The RON instruction does two things in my view. First it is a clear indication that the controller is no longer taking direct responsibility for the route that the aircraft is flying (and, consequently, for instructions which provide the requisite obstacle clearance). Second, it can be supplemented by instructions about the point that the FPL route is regained. Without this supplement the controller doesn't know what what the pilot will choose to do because various interpretations exist - do a search, the topic has come up before..

After a heading i´d either say "resume own navigation (to a point)" or "fly direct" (to a point), indistinctively. Spitoon, i know perfectly what the pilot will do, in both cases he/she´ll do the same, fly the shortest distance to that point, don´t you think?

Thanks, guys.

I just wanted to use less words.

If you specify the point that the pilot should fly to there's no ambiguity. My point was that unless you specify the point - and some controllers choose not to on occasion - you really have no idea where the aircraft will go.