Yes it is, or even if you're just a classical music fan who likes to listen while following along with the score.
I contribute $2 USD a month to Petrucci/IMSLP for the same reason I contribute $3 a month to Wikipedia and $11 a month to WNYC: for the greater good of human civilization.
I too contribute monthly to Petrucci. Apart from downloading the scores of piano pieces I want to play (they offer a wide set of arrangements for all but the most obscure pieces), I have discovered several hidden gem in their library, which grows more and more every time I check!
Before Petrucci, I used to be a regular user/contributor of the Mutopia project [1]. Its purpose is to provide Lilypond [2] source files for scores: this allows users to create MIDI files or re-create PDF files using custom page layouts/line breaks/etc. These features are handy, but creating a Lilypond file from a score is much more time-consuming than simply scanning it and uploading to Petrucci. (Each of Mozart's and Haydn's string quartet I uploaded took me ~2 weeks of work.)
