Since they cut off concession sales halfway through the third, I don't see why they care if people are leaving early. Those people aren't likely to stop at the merchandise stores on their way out either.
There are some people who consistently leave once the 50/50 number is announced whether it's a tie game or a blowout. If a tie game won't keep these people around, is anything going to?
I know at the Tampa game, a bunch of people near me jumped up at the end of the third to (presumably) make quick runs to the bathroom, but they were back before the puck dropped for OT.
A bunch of the others who leave their seats early probably watch the OT from lower seats or standing closer to the rink on the concourse. A lot of the newer arenas have unobstructed views of the ice from the concourse, so you can move closer to your exit and make a quick escape once the game ends.
In the past, before the dry scrape, they still did the shovel clearing of the ice, which took two or three minutes, so the dry scrape only adds two or three minutes to the break anyway, and eliminates an additional break between OT and the Shootout.
There are so many things that impact the length of a game, that the dry scrape seems pretty trivial, especially if it means better ice for OT. Look at the Flames. This year, they have only gone to OT twice at home so far, against Tampa and Montreal...
Here's the scoresheet from the Tampa game, which ended at the 3:00 mark of OT. Total game length was 2 hours, 28 minutes: http://www.nhl.com/scores/htmlreport...5/GS020088.HTM
Here's the Montreal game, which ended after three rounds of the shootout. Total game length was 2 hours, 48 minutes: http://www.nhl.com/scores/htmlreport...5/GS020134.HTM
Here are a couple of games last year that were longer than those two, despite ending around the same time on the clock...
Against Minnesota, the game ended at 2:25 of OT. Total length, 2 hours, 32 minutes: http://www.nhl.com/scores/htmlreport...4/GS020832.HTM
Against St. Louis, the game ended after three rounds of the shootout. Total length, 2 hours, 54 minutes: http://www.nhl.com/scores/htmlreport...4/GS020563.HTM
Actually, the last two games were about the same length as the Tampa game.