Offering OSS as Web services would validate the Web-services scheme as a means for commercial software vendors to collect monthly or annual SUBSCRIPTION fees for non-OSS software. That is a prospect over which software publishers have been salivating for years. The whole upgrade-or-else scheme to generate that cash flow and profits never really worked out, because too many stubborn SOBs like me looked at the bubblegum improvements in the upgrades and simply said, "Thanks, but no thanks." So they've been desperate to find another scheme. They've been watching the consistent cash flow and huge profits reaped by "content" publishers and thinking, "Geez, if only we could repackage our software as 'content', we could demand a subscription fee and make TONS of money." If Big Software can manage to "re-educate" people's perception of software, in the same way that, say, Big Pharma re-educates people about how to treat illness so that only their patented products seem viable (making people forget about folk medicine, etc.), then they'll win the war. Re-packaging software as Web services is actually Big Software's latest attempt at doing that, because Web services then "feel" more like content to people, and as we all know people are already indoctrinated to paying regular fees for content.
If OSS providers indulge in the same software-as-a-service route, it will validate that scheme and ultimately be handing a huge monetary victory to Big Software, a victory that will completely overshadow the small gains that OSS has made in recent years. Is that what we want?