Personally. I want to control what's happening, and use ctrl when I want things in a new tab, or even ctrl+shift if I want to go straight into the new tab. I do not like sites that just pops around.
I would also like to point out that I do not think the most popular geocheckers does this by default. Imo pretty much the same thing.
Finally, I will admit that all(?) external links from Project-GC opens in a new tab. This is a marketing decission, to keep users at the site. A quite common one.