ご指摘ありがとうございます。たしかにそうですね。
ヘルプには
 Defaults:
   --all
とあったのでオプションなしと同じかなとおもいつつ試しにやってみると両者は違うようです。
エラーになってしまいましたが...

% gem pristine -all
ERROR:  While executing gem ... (OptionParser::InvalidOption)
    invalid option: -ll

% gem pristine
ERROR:  While executing gem ... (Gem::CommandLineError)
    Please specify a gem name on the command line (e.g. gem build GEMNAME)

% gem pristine --all
Restoring gem(s) to pristine condition...
activerecord-1.15.3 is in pristine condition
mongrel-1.1.5-x86-mswin32-60 is in pristine condition
eventmachine-0.10.0 is in pristine condition
Building native extensions.  This could take a while...
cgi_multipart_eof_fix-2.3 is in pristine condition
activesupport-1.4.4 is in pristine condition
ERROR:  Cached gem for gettext-1.9.0-x86-mswin32 not found, use `gem
install` to restore
activeresource-2.0.2 is in pristine condition
windows-pr-0.6.2 is in pristine condition
rake-0.7.2 is in pristine condition
rake-0.8.1 is in pristine condition
ERROR:  Cached gem for win32-service-0.5.2-x86-mswin32 not found, use
`gem install` to restore
postgres-pr-0.4.0 is in pristine condition
cgi_multipart_eof_fix-2.5.0 is in pristine condition
ERROR:  While executing gem ... (Errno::ENOENT)
    No such file or directory -
C:/Ruby/lib/ruby/gems/1.8/gems/rubygems-update-1.1.1

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