Does a pleonasm give more precision or is it just redundant?
Does συν alone sufficiently convey the thought of what belongs together in time and place?
I think the usual answer would be: there's a large amount of redundancy in most language use, but the redundancy is rarely absolute. When we use a pleonasm we generally feel that the extra word adds something
to what we're saying. (Otherwise, why would we add it?)
Sometimes the use of a pleonasm helps to reduce or eliminate potential ambiguities. In principle, "the thought of what belongs together" could be conveyed by the use of the dative alone, without any preposition at all. Whether or not Heb. 11:11 really is to be construed as αὐτῇ Σαρρα "together with Sarah," at any rate there would be no grammatical objection to that reading of it (BDF §194(1)). So, strictly speaking, even the use of a σύν would be a pleonasm. But the dative has so many potential uses that a supplementary preposition would generally help to clarify the sense and rule out other possible readings. Σὺν αὐτῇ would be less easy to misconstrue than simply αὐτῇ. And (at least in some contexts) ἅμα σὺν αὐτῇ might be clearer still.
Another possible reason for using a pleonasm can be to give extra prominence or emphasis to a thought. This could have particular relevance to 1 Thess. 4:17, where the writer is stressing as strongly as possible the absence of any separation (see verses 14-15).
Personal taste is also a factor. In Greek as in English, some writers just like to be more pleonastic than others.
Does the adverb ἅμα modify ἀρπαγησόμεθα while συν qualifies αὑτοῖς?
Different people would give different answers to that. Thayer says yes: "In 1 Th. iv.17 and v.10, where ἅμα is foll. by σύν, ἅμα is an adv. (at the same time
) and must be joined to the verb" (p. 30). Robertson, however, reads the construction as "ἅμα σύν with the instrumental" and construes it as "a sort of double preposition" (p. 638).
The fact that it's also possible to write simply ἅμα αὑτοῖς (Matt. 13:29) would tempt me to side with Robertson. It's difficult to think that the insertion of a σύν into Matt. 13:29 would transform the function of its ἅμα.