Social interaction is just one example; I picked it because it is the most common. That said, I would argue that making a character with zero social skills (in order to put those points into more combat) would restrict you to a fairly narrow subset of roleplaying opportunities.
I disagree. (As a side note, it's actually not very easy to put skill points, specifically, into "more combat", with a couple of exceptions.)
Is your character a self-appointed prophet ? Well, then you should probably take "Spell Focus: Divination", as opposed to something more battle-worthy.
No, I don't agree with this at all, even granting the premise that you have decided to pick feats/etc. based on story. Unless, of course, you look only at the barest surface features of the options you're selecting, without delving even a bit deeper.
Spell Focus: Divination is a feat that sounds like it generally "makes you better at divination" or some such. What it actually does (as you know) is make those of your divination spells that have saving throws harder to resist. Now, since most divination spells have no saving throws (certainly those spells most appropriate to prophecy, which are presumably most central to your character concept, do not), picking SF:Divination doesn't make any sense mechanically. Would your character "select this feat" (i.e. work toward developing this capability)? I don't see why. It's not like the "feat" has this name "in-game-world"... and your character presumably knows the "feat" by what it actually does. Why would he work toward an ability that has nothing actually to do with his chosen vocation (prophecy)?
So in this case, selecting Spell Focus: Divination would only make sense if you were trying to build a character that, on casual inspection, sounds like he would be a good prophet. It would not make sense if you were trying to build a character who was actually good at prophecy, game-mechanically; nor, on the other hand, would it make sense if you were trying to "organically" build a character who, in-character, tried to be a good prophet.
Is your character a skilled craftsman ? Then you should take a bunch of item creation Feats instead of going deeper into the "Power Attack" tree.
This likewise makes very little sense, not least because it's a rare character indeed who both would normally select Power Attack and its descendants (a selection usually made by martial characters), and would even qualify for item creation feats (which require one to be a spellcaster). Furthermore, item creation feats give you the ability to create magic items, i.e. imbue items with magic power; they have little actually to do with being a skilled craftsman, as such (that's better handled by the Craft skill). Once again, you would only make this connection if you were going by shallow surface associations. (You may, of course, interpret a feat such as Craft Magic Arms & Armor as representing your skill at crafting the arms and armor in question, even in the absence of ranks in the relevant Craft skills. In such a case, my first objection applies. If, in fact, you are a martial (or semi-martial) character who does nonetheless qualify for item creation feats, then that means that they are probably not actually bad choices for you. That's how the game is structured. Craft Magic Arms & Armor, by the way, is in most cases a more effective feat than Great Cleave is.)
Were his parents abducted by crab-people ? Then the crab-people are probably your racial enemy, despite being incredibly rare.
This doesn't really follow; it may make sense, but then again it may not. The reasons why it doesn't make sense have more, I think, to do with how story and world building is shared between the DM and the players, and so I think I will eschew this particular tangent unless you ask me to elaborate.
Thus far, I take your comments to be evidence against the claim that there are meaningful tradeoffs between roleplaying and effectiveness, not for it.
There are things that are worthless-- that provide no value. There are also things that are worse than worthless-- things that provide negative value. I have found that people sometimes confuse the latter for the former, which can carry potentially dire consequences.
One simple example of this is in fencing. I once fenced with an opponent who put a bit of an unnecessary twirl on his blade when recovering from each parry. After our bout, one of the spectators pointed out that there wasn't any point to the twirls and that my opponent would improve by simply not doing them anymore. My opponent claimed that, even if the twirls were unnecessary, at worst they were merely an aesthetic preference that was useless but not actually harmful.
However, the observer explained that any unnecessary movement is harmful in fencing, because it spends time and energy that could be put to better use-- even if that use is just recovering a split second faster! [1]
During our bout, I indeed scored at least one touch because my opponent's twirling recovery was slower than a less flashy standard movement. That touch could well be the difference between victory and defeat; in a real sword fight, it could be the difference between life and death.
This isn't, of course, to say that everything unnecessary is damaging. There are many things that we can simply be indifferent towards. If I am about to go and fence a bout, the color of the shirt that I wear under my jacket is of no concern to me-- but if I had spent significant time before the bout debating over what shirt to wear instead of training, it would become a damaging detail rather than a meaningless one.
In other words, the real damage is dealt when something is not only unnecessary, but consumes resources that could instead be used for productive tasks. We see this relatively easily when it comes to matters of money, but when it comes to wastes of time and effort, many fail to make the inductive leap.
[1] Miyamoto Musashi agrees: