As @P.E.Dant writes, dropping it is ellipsis. So if either option is less careful, it would be dropping, not adding, the second verb. (That said, both are very common in every register of speech.)
The "redundant" option is often used to avoid ambiguity, because the other side of the comparison could be the subject or the object of a previous verb.
Strong Sad of Homestar Runner says at one point:
I like board games more than most people.
— Strong Sad's Character Video (wiki page with transcript) (video)
Interpretation (1): "than" relates "most people" and "I", making "most people" a subject. That is, both Strong Sad and most people have an opinion of board games.
Interpretation (2): "than" relates "most people" and "board games", making "most people" an object. That is, Strong Sad has an opinion both of board games and of most people.
Strong Sad goes on to say that both of those are actually true in his case:
(1) By that I mean I like to play board games more than most people do.
(2) But by that I also mean I like board games more than I like most people.
In clarification (1), he repeats "like" by adding "do", which would be redundant except that it forces "most people" to become a subject of the verb. (The added "to play" doesn't affect this.) So both Strong Sad and most people have an opinion of board games.
In clarification (2), he repeats "I like" before "most people", which would again be redundant except that it forces "most people" to become an object of the verb. So Strong Sad has an opinion of both board games and most people.
But unlike Strong Sad, most people usually mean only one of the two ambiguous interpretations. :) So we use a redundant verb to resolve the ambiguity.
It's a fairly common situation in everyday life, but most of the time the context makes the speaker's meaning clear, and we perform the ellipsis. When it's unclear, we repeat the verb.
Oddly enough, sometimes we supply the redundant verb even when one of the readings is semantically improbable, as in your example: it doesn't make sense for "Robert" to be the object since he's not a measurement of speed (compare "He runs faster than 50 km/h"), so Robert must be the subject of the (implicit) second "runs". I guess the syntactic ambiguity makes us err on the side of caution.
Edit: @Muzer makes the point that some pronouns in English are cased, e.g. "I" (subject) vs. "me" (object). In theory that's enough to distinguish the two interpretations for those pronouns. That said, s/he also mentions prescriptivist grammars and I think there's something to that. The average speaker uses either "than me" for both meanings, or "than me" vs. "than I do". Ending the sentence at "than I", economical though it may be, can sound somewhat pretentious!
P.S. Don't attach too much importance to anything Strong Sad says in that video. :) That said, Homestar Runner can be a goldmine for linguistic curios if you're nearing fluency. They often play with very subtly odd uses of English — or downright outrageous ones, which can be hilarious.