*sigh*
There is Progressive as a noun, which is the name of something and as a general rule the first letter is often capitalised to emphasis that it is the name of something specific rather than the name of something generic (such as cat for example as opposed to Tiddles the cat) - as with all names it is perfectly acceptible to shorten that as a diminutive, hence we get Prog, which is still a noun and is still the name of the same something.
And there is progressive as an adjective, this is a 'describing word' that we put with a noun to describe, modify or other substantive by limiting, qualifying, or specifying characteristics of the noun, as a general rule unless it is used at the start of a sentence (which would be difficult to do in correct grammar) the first letter of the adjective progressive should never be capitalised. Also, progressive as an adjective should not be shortened at all (ever) as it is the -ive suffix that denotes that it is the adjective form of progress, remove the -ive ending and the word looses its descriptive meaning completely.
Therefore when you describe a tune as being very Prog, you are saying that it belongs in the genre of music known by the noun Prog or Progressive - you are not saying that it is a piece of music that has progressed.
Therefore it is grammatically correct, for example, to describe an artist as a progressive Progressive Rock artist since we are using both the adjective form and the noun form of the word.
Therefore it is musicologically correct for a band or artist to belong in a genre of music known as Progressive as a name (as in Progressive Rock, or Progressive Metal) without their music being described as being progressive as an adjective.
Armed with this basic knowledge of the usage of Progressive as a noun and progressive as an adjective any further discussion on this subject is rendered pointless as any point, comment or opinion can be systematically categorised as using the word progressive either as a noun or an adjective.
Edited by Dean - May 01 2013 at 19:14