How can I automatically add/update Depends/Imports

I like to keep my R packages up to date, and in developing my own package, I want to stick to @Hadley's advice:

Generally, it’s always better to specify the version and to be conservative about which version to require. Unless you know otherwise, always require a version greater than or equal to the version you’re currently using.

So, I'll need some of those here in DESCRIPTION:

Imports:
 knitr (>= 1.13),
 rmarkdown (>= 1.0)

Is there an existing tool to programmatically update the versions of dependencies based on the packages I currently have installed?

I know this is a small thing and I can do this manually, but I just know this is the kind of thing that is easily forgotten.

Using the locally installed version of a package as a minimum dependency also seems to make sense because I tested / build with those dependencies.

Is there a reason why I shouldn't do this?

This shld do what you ask (well, you still need to cut/paste the output into DESCRIPTION :-)

#' Add curent version string to package dependencies
#'
#' Will \code{cat} out a cut/paste-able set of fields for a
#' \code{DESCRIPTION} file with minimum required versions for
#' each package based upon currently available package vesions
#' in CRAN.
#'
#' @param pkg package description, can be path or package name
#' @param fields fields to get & report dependencies for
#' @note R and the R version is NOT added to \code{Depends}
#' @examples
#' add_pkg_versions("qmethod")
#' add_pkg_versions("MASS")
#' \dontrun { # assumes you're in a pkg devel dir
#' add_pkg_versions()
#' }
add_pkg_versions <- function(pkg=".",
                             fields=c("Depends", "Imports", "LinkingTo", "Suggests")) {

  require(purrr)
  walk(c("dplyr", "tools", "stringi", "devtools"), require, character.only=TRUE)

  stopifnot(is_scalar_character(pkg), pkg != "")
  fields <- match.arg(fields, c("Depends", "Imports", "LinkingTo", "Suggests"),
                      several.ok=TRUE)

  avail <- as_data_frame(available.packages())

  if (pkg == ".") {
    pkg_deps <- unclass(as_data_frame(read.dcf(file.path(package_file(), "DESCRIPTION"))))
    pkg <- pkg_deps$Package
    map(fields, ~stri_split_lines(pkg_deps[[.]])) %>%
      map(function(x) {
        if (length(x) > 0) {
          unlist(x) %>%
            stri_replace_all_regex(" \\(.*$|,", "") %>%
            discard(`%in%`, c("", "R"))
        } else { x }
      }) -> pkg_deps
    names(pkg_deps) <- fields
  } else {
    pkg_deps <- map(fields, ~flatten_chr((package_dependencies(pkg,  which=.))))
    names(pkg_deps) <- fields
  }

  pkg_deps <- discard(pkg_deps, function(x) {length(x)==0})

  map(pkg_deps, function(x) {

    non_base <- filter(avail, Package %in% x)
    base <- setdiff(x, non_base$Package)

    non_base %>%
      mutate(pv=sprintf("%s (>= %s)", Package, Version)) %>%
      select(pv) %>%
      flatten_chr() -> pkg_plus_version

    sort(c(pkg_plus_version, base))

  }) -> pkg_deps

  cat("Package: ", pkg, "\n", sep="")
  walk(names(pkg_deps), function(x) {

    cat(x, ":\n", sep="")
    sprintf("    %s", pkg_deps[[x]]) %>%
      paste0(collapse=",\n") %>%
      cat()
    cat("\n")

  })

}

One of your packages:

add_pkg_versions("qmethod")

Package: qmethod
Imports:
    digest (>= 0.6.10),
    GPArotation (>= 2014.11-1),
    knitr (>= 1.13),
    methods,
    psych (>= 1.6.6),
    tools,
    xtable (>= 1.8-2)

Just to show edge cases are handled:

add_pkg_versions("MASS")

Package: MASS
Depends:
    graphics,
    grDevices,
    stats,
    utils
Imports:
    methods
Suggests:
    lattice (>= 0.20-33),
    nlme (>= 3.1-128),
    nnet (>= 7.3-12),
    survival (>= 2.39-5)