Although I like Thaddeus' answer, I'm not sure the OP was referring to the "mega-crossover" books.
So, if I guessed correctly:
No, there is no defined point when this change in storytelling occurs, it varies saga to saga.
The Phoenix "Saga's" are stories that would suffer under non-XMen intervention. It's a different kind of story.
XMen and other Marvel titles have always enjoyed frequent crossovers, but when entering a saga that a crossover would be a detriment, they keep them out.
Captain America and Thor showing up in the Phoenix Saga would have diluted the personal dynamics, and (in my opinion) would have made the struggles "a bit silly".
Even in the very early Uncanny XMen of the 60s there were significant crossover appearances of the Avengers and Fantastic 4's.
In addition to there being frequent crossovers since the 60s, I'd like to point out an other reason for the gradual rise is crossovers in general (besides Mo Money).
In the earlier days the use of crossovers was mainly a crutch to wean people onto a series. The X-Men enjoyed mentions of various Fantastic 4's which gave people a quicker understanding into characters like Angel, who was often paralleled to The Human Torch.
As time went on, and more and more characters were established by Marvel there became more reason for crossovers due to the diverse character sets. As more characters are added, more of them share parts of their past or locations (just as a function of diversity, eventually an X-Man showed up who was from Captain America's generation, etc).
Most of the Marvel universe is made up of groups of people, heroes, villains, and misfits.
It only makes sense that as that universe grew that parts of it should logically overlap, and that can be reflected in the storytelling.