I face this decision almost every day. Sometimes is easier, sometimes is harder, but there are certain rules that can help.
Like Tohster says, demographics plays a big role, and as a matter of fact is the keyword in this decision.
However, I kind of disagree with the figures of that netmarketshare link. First, because they're nowhere close to anything I can see, and we closely monitor 100+ sites in all kind of niches and demographics. Then I realize the link is only about desktop, which means a HUGE difference
Also, there's an evolution in stats. Take this graph showing browser usage for last 12 months:
as you can see, IE8 accounts for 2.12% of browser usage.
Not a big deal, but at least something to consider.
However, this takes the period starting June 2014. In August 2014, Microsoft announced end of support for XP AND the end of support for IE8 on January 2016. Now take a look to IE8 usage last 3 months:
Interesting, huh? IE8 is virtually unused. And another thing you can test by yourself: IE8 users are used to see weird pages and they don't care at all, because they're aware of the obsolete system they're using and because most of them use another device as well (another desktop, mobile, tablet, etc)
Now, you can run happy knowing that IE8 is not needed at all, but this is not completely true. Take a look at last 3 months in US:
yep. 3.23% still using IE8. In a technologically advanced country
Now, you can think US is running under a technology involution, but reality is very different: US has the biggest amount of companies, universities (and even military forces) running Windows Server with IE8 apps, and this is not as easy as to change a browser. Right now there are thousands of companies in the process of doing this upgrade, so it will change in a few months.
NO to IE8 if...
....your audience target is worldwide and for a generic product/service, don't worry about IE8 (unless you have the time or will to do it).
YES to IE8 if...
...your audience includes big companies, corporations , education and/or military, then yes, try to work some solution for IE8.
And finally, the easiest, easiest, easiest advice to follow: track your audience and check what they're using!