programming - Is there a practical way to define a default value for missing keys in an Association?

Clarification

Although I may be missing his point I currently feel that Leonid's comments below are misleading. I am not looking for anything that is not already a part of Association functionality other than a way to define what is returned for a missing key on a per-association basis. I thought that the Block example below made this clear but perhaps not.

I am seeking a way to do something like this:

asc = <|"a" -> 1, "b" -> 2, _ -> 0|>;


asc["x"]

(* desired output: 0 *)

(* actual output: Missing["KeyAbsent", "x"] *)

Critically I am not looking for general pattern matching of Key names however; I only want a way to define one default value for missing keys.

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Original ramblings

It can be very useful to define background or default value for an object that can be incremented or otherwise modified. A simple (and for me, common) use is a counter:

count[_] = 0;

++count[#] & /@ {"a", "a", "b", "a", "b", "a"}

{1, 2, 1, 3, 2, 4}

Since there are advantages to Association such as ease of copying and direct manipulation of keys and values I would like to port this method to new function. However I cannot think of a clean, practical way to define a default. (I consider directly overloading System functions such as Increment undesirable.)

I note that it is possible to increment a missing value and then clean it up afterward:

asc = <||>;


++asc[#] & /@ {"a", "a", "b", "a", "b", "a"} /. _Missing -> 0

{1, 2, 1, 3, 2, 4}

This isn't really the same as setting a default value however and it limits the way this method can be used. Somewhat better I think is to temporarily redefine Missing. This at least gives values that are up-to-date while the operation is performed.

asc = <||>;

Block[{Missing},

  Missing["KeyAbsent", _] = 0;
  ++asc[#] & /@ {"a", "a", "b", "a", "b", "a"}
]

asc

{1, 2, 1, 3, 2, 4}

<|"a" -> 4, "b" -> 2|>

This could be packaged a bit more nicely but it still feels like a bit of a kluge.

Is there an approach I am failing to consider? Or by some slim chance is there a hidden way to do this?