gistfile1.txt

If only one of the text fields are filled in the params will look like this:

  {"foo" => "bar"}

While it looks like this if both are filled in:

  {"foo" => ["bar", "baz"]}

And really, in the first case it *should* be:

  {"foo" => ["bar"]}

While there might be some debate over this, I think it's pretty natural:
The code that handles the form data expects params["foo"] to be an array.
With the current behaviour that code needs to check wether params["foo"]
is an array or something else, and then act differently depending. If
params["foo"] always is an array, the same piece of code handles it all.
Less hassle.

nested-params.rb

require 'rubygems'
require 'sinatra'

get '/' do
  puts params.inspect
  haml :index
end

__END__

@@ index
!!!
%html
  %body
    %form{:action => '/', :method => 'GET'}
      %input{:type => 'text', :name => 'foo[]'}
      %input{:type => 'text', :name => 'foo[]'}
      %input{:type => 'submit'}