pros/cons of teaching attach()

I think that the reference to not using $ or [[ was meant for cases where there is a proper extraction function, residuals being the example used.

If "fit" is an lm object then I could plot the fitted vs. residuals plot like this:

plot( fit$fitted, fit$residuals )

Or like this:

plot( fitted(fit), resid(fit) )

The second one is preferred as it properly extracts the information without needing to know the exact contents of fit (and the axis limits look a little nicer).

With an lm object the 2 plots will be essentially the same, but what if fit is a glm object?  Then fit$residuals does give something that fits the definition of residuals, but of the different types of residuals available for glm fits, this gives the one that is least interesting/interpretable to humans (they were useful to the program for getting the fit).  Here the resid (or residuals) function defaults to a more meaningful set of residuals and gives options for other types.

If we have arbitrary objects without extractor functions then we need to use $ or [[ to extract the individual elements, but when working with fitted objects it is much better to teach students to use the proper extractor functions rather than directly working with elements of the object itself.