Here are a couple of reasons I really like this new feature:

• The Adwords ads need a slight tweak in appearance to get back on people's visual radar. After looking at the same look, feel, and format for so long searchers eventually put their blinders on to the ads. This might catch some attention and give some new life to the ads.
• This format ads a little more incentive to grabbing that top spot. That addition, in my opinion, makes it far more likely that a searcher will see and interact with your ad.
• Possibly the most important factor is that the advertiser is NOT charged if a searcher opens the PlusBox and grabs a phone number and driving directions. You're only charged if the searcher clicks on your ad and comes to your site. This could save local advertisers some money while still enhancing the experience for the searcher.
• Another benefit is that advertisers will be encouraged to localize their ad efforts where possible which I think makes the search engine user's experience all that more efficient.

The only real drawback I see is the effect this has on other advertisers on the SERP who don't get that top spot. This has the potential to lower click-through-rates unless you're able to either improve your Quality Score or bid higher to get into that top spot.