First of all, the technical part - if this bores you, just scroll down to the bolded line below. I won't think any less of you.
To see how important an incumbent is, we need a way to calculate an expected result. After spending a lot of time tinkering with various projection models, I've come to the conclusion that a simple arithmetic transfer of votes works the best - at least for the 2006 and 2008 election, which is what I'm looking at (if you want to see residual charts and regressions and such to prove this, just e-mail me). So I carved the country into 30 geographic regions and projected accordingly - if the Liberal vote dropped 2 points in Calgary last election (it did!), I subtracted 2% from the 2006 result from every Calgary riding.
How accurate is this? Well, half the results fall within +/- 2% of the actual result, with 90% falling within +/- 6%. The predicted values correlate 96.3% with the results. Keep in mind, we don't expect the results to be spot on because, after all, the candidates and campaigns should make a difference.
For those interested in the fine print, by election winners were considered to be incumbents, so Bob Rae was an incumbent MP last election. And Garth Turner became a Liberal incumbent, with Wajid Khan as a Tory incumbent (for what it's worth, Turner's impact was negligible, while Khan, not unsurprisingly, undershot the expected Tory vote by 3%).
So what does the data show?
For both 2006 and 2008, in ridings without an incumbent, the incumbent party took a 4.1% hit compared to their expected result. Since the totals have to sum to zero, that means incumbents performed about 1% above expected, for an overall benefit of around 5%. These results were largely identical both elections and mesh well with the academic literature on the subject so I don't have any reason to doubt them.
What's also interesting, as an aside, is that MPs were more likely to call in quits in regions where their party was heading for a drop in support (think of the Tory Newfoundland MPs before last election). So "spending more time with my family" loosely translates to "I can see the writing on the wall" quite often.
Now, I do think it's important not to read too much "cause and effect" into this. As Andrew Steele comments here (citing freakonomics...a book I adore), incumbency effects are often overblown once other factors are controlled for. And when we're talking about "incumbency", we're really talking about resources, organization, and "name recognition" - Gerard Kennedy and Justin Trudeau weren't incumbents last election but I'd wager a lot more people knew who they were in their home ridings than many obscure backbenchers (I'm looking at you Brian Jean). Quite simply, not all incumbents are created equal. But a 5% swing is nothing to sneeze at - 41 MPs were elected by smaller margins than that last time.
So, once MPs start announcing their retirement, we should be paying attention. Because if they're running in ridings with a slim margin of victory, it certainly could put those seats into play.