To respond to
fondue's points...
Granted, the plot of
Back to the Future Part II uses a more developed
time-travel theory than its prequel. The most common gripe/plot hole is that
Biff Tannen is able to return to the 2015 from which he left, even though he radically changed both his life and the landscape of
Hill Valley. The common response is that events are not changed instantaneously, a theory which is not disproven anywhere in the series.
Indeed, the scene where
Biff returns to the future was cut short--he exits the
Delorean in great pain, staggers behind a dumpster, falls to the ground (apparently dead), then slowly fades away, as if "
erased from existence." This part of the scene was cut, deemed too confusing; however, it seems to confirm this theory, that a radical shift to another
parallel universe takes some amount of time.
As for Doc's apparent hypocrisy...
Doc Brown drops Marty off at home near the end of Part I. He immediately travels to the future; what he did there we can only speculate. Most likely he tried to see what Marty was doing in the future and was dismayed to find Marty a failure and the entire
McFly family in ruins.
He discovers the ultimate cause of this: Marty gets in a
car accident in 1985 (spurred by his refusal to back down from a challenge), injures his hand, and effectively wrecks his
music career. Jennifer feels sorry for him (parallel with Marty's parents) and marries him; fast forward to 2015, and Marty is stuck in middle management, his children up to no good, and his wife a drug addict.
Doc remembers how Marty changed history to save Doc's life, even though it could potentially have done damage to the
space-time continuum. Marty is perhaps Doc's only friend; and so Doc wants very much to fix things for Marty, but knows that to explain all this to him would be dishonorable from a scientific standpoint, and potentially damaging to the universe. However, he knows that to prevent the jailing of Marty's son is
damage control, so he allows himself to tell Marty about it and alter the course of events for a good cause.
The reason Doc gets so mad at Marty for purchasing the
Sports Almanac is exactly what he says--he didn't invent the time machine for financial gain. His alteration of time was good-intentioned; Marty wants to alter time to make himself rich.
Of course, later in
Part II Doc admits that travelling through time in general is a horrible idea; and that with the small amount of travelling they've done, Doc and Marty have done plenty of damage.
As for the movie's portrayal of the future, of course it's rather dated. An accurate depiction of the future would require a good amount of clairvoyance, and I'm sure that a genuine portrayal was not the intent in the first place. The movie projects 2015 as having grown straight out of 1985, thus all the
product placement.
Texaco is still around, yes, but there's no implication that they're still in the business of
gasoline--nor is there the implication that
fusion technology now powers
all cars, rather than just being a luxury of the rich.