Memory usage is only pleasant a side effect. The real reason I'd recommend using Option Explicit is that it allows the compiler to protect you from coding mistakes that compile and run, but don't do what you intend.
Example 1: It keeps typos from turning into unexpected disasters.
Dim variableNamedFoobar As String
variableNamedFoobar = "Something"
If varableNamedFoobar <> "Something" Then
Debug.Print "Did you catch that? The compiler would..."
End If
If you aren't explicitly declaring variables, the compiler happily gives you a 22 byte empty string to compare against.
Example 2: It also protects against leaking scope. Consider a huge project with multiple levels of variable scope. Hope you remember what your globals are:
Private x As Integer
Private Sub First()
'I remembered here that x is a global.
x = 2
Debug.Print x
Second
Debug.Print x
End Sub
Private Sub Second()
'I forgot here.
For x = 1 To 5
Next x
End Sub
Adding Dim x as Integer for use as the loop counter ensures that it is scoped to Second().
Example 3: It allows the compiler to detect type mismatches.
Set foo = New Collection
foo.Add "Bar"
Debug.Print TypeName(foo)
'... 100 lines of code later...
foo = 6
Debug.Print TypeName(foo)
If you had Dim foo As Collection in there somewhere, you get a compile time error letting you know that you already had a foo and you shouldn't be assigning 6 to it.
There are lots of other examples of where you can shoot yourself in the foot (or higher) with implicit variable declarations. While these examples are easy to spot in the isolation of the code blocks above, any one of them can cause subtle and extremely difficult to debug errors in a large code base. Do yourself (and everyone that might need to maintain your code later) a favor and type the extra line of code.
