Yes, there's no way to get the other matches using [] as far as I know, it's just a generalization of being able to pass various types to it; Fixnum, Range, Regexp.. I don't see the harm in doing something with String too. If you wanted to extract multiple items, you'd use something like String#scan, or if you wanted to do a global replace, String#gsub, which can also accept a code block to perform computed replacements.
I can see where standardized methods like to_a can be more convenient, but they also open up the possibility for unlimited ambiguity. What does Foo.to_a do for your Foo class?
As of 1.9, it does this:
NoMethodError: undefined method `to_a' for Foo:Class
In Python, if I do `[foo]` Then foo is now a list.
You do the exact same thing in Ruby, though if you're wanting to cast your argument like the old to_a did, you use Array(), which won't turn [] into [[]]. If you want a number, Integer() or Float() will raise an exception if they can't give you one.
A lot of Ruby's conventions simply scream "ambiguity" to me. A "subset of a collection" is extremely ambiguous; if there is more than 1 foo in my_string, does it always get the first? How do I get the rest? my_string['foo'][2] or something? I can't even intuitively make something up which makes sense to me. Perhaps there isn't a way to get the others, which I suppose would be in line with Python, which doesn't supply a string method to find all occurrences at once... but how do I offset it?
I can see where standardized methods like to_a can be more convenient, but they also open up the possibility for unlimited ambiguity. What does Foo.to_a do for your Foo class? It could do anything! There's no way I could rely on these methods to do something across the board unless I know your class conforms to some Interface (do Ruby folks use/have Interfaces?). In Python, if I do
Then foo is now a list. I always know what I'm getting. I always know what int() is going to attempt to do to an object, etc.