I know the differences. I know that there are around a dozen (maybe more) users here on this site who are undecided. I am one of them.
We'll see about that.
"oh well I'm 50/50 S/N" <<
Bloody tests.
It's a recurring theme on this board and others.
Ignorance is infectious.
Why do INTPs and ISTPs have to be different?
4 dichotomies result in 16 individual types, not 15. Doh.
We are the Ti-doms. 
'The presumers' you mean?
It's been said that one way to quickly tell the difference is that
ISTPs care about how things work - that is often the purpose of (our/their) logical analysis.
Not specifically, no.
To be interesting, the object of analysis must be practical and concrete.
Not specifically, no.
ISTPs use known data and facts to reach underlying principles. Is this where Ni comes into play?
Underlying principles are judged by Je, perceived by Pi. It's not function related. Besides, associating the ISTP with underlying principles made me lol. Whahahaha.
So, do INTPs not care about how things work? Are (we/you/they) really just interested in theories for the sake of theory?
I'll leave this open for INTPs to answer.
Is this what makes (them/you/us) pure thinkers?
Pure thinker would imply what?
INTPs can analyze anything, concrete or abstract? Is that it?
No. INTPs can not analyze abstract things. Nor can they analyze concrete things, lol. They analyze judgment, not perception! All the EPs and IJs are analyzers of perception. (IJ = Context analyzers, EP = Content analyzers). The IP (e.g. INTP) merely observes judgment, it is both their weakness and their power. Everything resolves around Ji. Ji is subjective judgment (Subjective opinion). So the very basis of their thought, is to serve subjective opinion. An opinion which they would argue factual for the sake of Je (Inheriting objective opinion).
So let's talk about Se, then? It's so easy to work yourself into a never-ending loop:
Se notices the details in the environment and may lead to spontaneous action,
True.
ISTPs have a tendency to conserve their energy, biding their time until they "sense" something that draws their attention, then they take action.
This is written VERY vague. Let me explain it a little. ISTPs don't conserve their energy for the sake of it. They conserve their energy because everything resolves around Ji, ergo they do not want to make Directive (Evironment-Wise) action. They merely judge, judge and judge things internally untill they are 'forced' out.
"until they "sense" something that draws their attention". Yes, their sexual attention. Orwell that might be to harsh to say. Anyhow, my point is... Is that the ISTP is very obvious. When you see an ISTP take action, you'll know with easy for what purpose (at least provided if you would be an INFJ).
ISTPs have exceptional hand-eye coordination
Whahaha, they don't. It's mildly better than the INTP, but FAR worse than the INFJ or ENFP.
... respond quickly and accurately in emergency situations
Quickly, yes. Accurate? No. Accuracy is not their concern.
, Se gives them a leg up on the physical world, talented athletes ...
Not particularly. The ISTP is rare in the athlete world (compared to the INFJ).
But there are lots of examples of ISTPs who defy this and INTPs who defy (by proxy) it as well. Once again, we get nowhere. Most people are halfway in the middle.
That is because of ignorance. The IxTP have an utmost ignorance in how context relate to context. They take everything out of context by nature and purpose. As I said, being Ji dominant is both their weakness and power.
It's a Bell Curve meets Venn Diagram situation and the probability that a given individual is not a
"pure" type seems to be very high.
They say that the auxiliary function isn't really developed all that well until late adolescence?
Who told you this bullshit? lol.
So until then, what? INTPs and ISTPs are too similar to differentiate?
INTP and ISTP is REALLY easy to differentiate. As are all S/N.
WE NEED STEREOTYPES people!
No you don't.
Then there's video typing - the end all. Or is it? Suddenly we can tell the difference between Ne and Se in thirty seconds by watching a person's eyes. Now there's "pure" INTPs and "mudblood" INTPs ... INTPs who actually aren't INTP because their eyes don't reflect TiNe. So what then? We're at the mercy of the video typing experts?
What video typers type, has great depth. Some don't know this though, and try to type people using the techniques. They suck at it. (e.g. Auburn)
The oft' stated advice to newcomers is to perform a functional analysis of the self.
Identify and understand the 8 cognitive functions and their respective polarities, learn how these correspond to the 16 MBTI types.
Ti-dom, check
Fe-inferior, double check
Ne, wtf is that?
Se, ...uhhhh check
Si, check, definitely!
wait ... contradiction already? OK, so NOT Se.
Te? sorta
Ni ... mayhaps
oh well let me just take the cognitive function test
It says I use Se and Si? hrmmm.
It also says I use Te and Ti.
INFJ? No ... wait, they're too rare. No, probably INFJ. Everybody is INFJ these days...
ISTP then! No ...
Si (D'OH) Maybe I don't "get" Si.
How come every Si description available rings home then?
I must be Si-dom?
ISFJ then. ... well, no not really. That's not right...
Can't I be TiSi? No? DAMMIT.
INTP then ... wait, no ...