I would be fine with Petrovski for generally the same reasons you stated. I would also be fine with signing one guy like him each year; i.e. your 18-year-old undrafted camp players.
The practice of signing guys in that position, though, I wouldn't make it a major piece of my prospect acquisition strategy. The contract may not count up front, but it does count eventually, and then you're most likely stuck with a bunch of crappy guys that maybe fill out your ECHL team. There's so much unknown with these players and that I think is one reason you don't see a ton of guys being signed under these circumstances. Also the CHL leagues, particularly the OHL and WHL, are so heavily scouted, it's really rare for a guy from those leagues to slip through the cracks. If he isn't drafted there's usually a good reason.
I think the better bet is signing the traditional UDFA players particularly from the NCAA ranks or Europe. You lose the benefit of the contract sliding but gain those extra years of development and some additional "certainty" about what a guy is or isn't.