Following the interesting debate in this thread, I find myself agreeing with the aspects of a good slot's role as you describe them. Your last 2 sentences describe the slot role -- and the reason it is not an easy one to fill by many teams -- best, in my opinion.
In fact, this challenge was confirmed by Gilbride at the beginning of 2011 season, when asked who was going to fill the Smith role after we all learned of Smith's departure. As of Week 3, prior to the game vs Eagles, he admitted he still had no answer and was still in the process of determining who--if any--on the team was capable of stepping into that role. In fact, it was the reason we picked up Stokely--but then he was quickly injured.
At the time, KG was thinking either Hixon (who went down for the season in the first half of game 2) or Manningham (who was concussed in the same game) were his best chances of developing at least an adequate replacement for Smith.
And the reason he gave for it being so challenging a position to replace was that it is a much more complex position to play well consistently. The reason for this is that it requires not only certain physical attributes (primarily quickness and instinct), but it requires the ability to read the defensive formations in their entirety as it develops after the snap, and to instantly telegraph to his QB his resultant move, much more so than any outside receiver is required to do.
I love what Nicks brings to our receiving corps. He brings more than any Giants receiver I can remember, going back to 1980 (and I was a huge fan of Toomer when he played, but Nicks will surpass all that he did for us, imo). Even though he had a fall off year this year, I see that as aberration due to injury and he will return to form next year and beyond. I believe he is comparable to Fitzgerald and in some ways to Michael Irvin in their respective abilities a "the Playmaker".
But Cruz is a Welker with more height and speed. And, unlike Welker who is 6 years closer to the end of his career, Cruz is just beginning and has shown the ability to learn from mistakes and improve. And he can function at a high level on the outside when asked.
Much--on this board--has been made of his drop to 1100+ yards in 2012 from 1500+ yards in 2011. I think this is very narrow thinking. A superior slot/wide receiver is more valuable to a team than is measured in total yards for a season. And after 2 complete seasons, Cruz has--to most fans at least--proven he is more than any "one hit wonder".
Fact is, if we want to continue to be more like the Giants offense of 2011--and I believe we will--we need both Nicks and Cruz. They embody the phrase "the sum is greater than the parts".
Because of that, in my opinion, Reese wants to find a way to keep Cruz in this off season, and equally wants to find a way to keep Nicks next off season. It will not come down to "either / or" as some on this board fear.