Entrecard does help increase your blog traffic. So, if you see it from this side, it does help your blog. But then, what kind of traffic do you want? After a few weeks experiment installing Entrecard widget on my blog, it increases my bouncing rate since apparently, many Entrecarders just visit sites, drop cards and leave. It's not necessarily bad, though. If you make your 10 last posts visible at one glance, then they might just read it on your blog main page and leave without clicking any links or posting comments.
If you don't care about statistics, adding EC widget this way won't hurt.
At this point of view, Entrecard does help your blog. It also has Contest system that help boosting your blog's popularity in short period of time. Very useful to help getting traffic in instant. The drawback is, Entrecard does distract you (even if it's just a bit) from your blog since you need to drop cards to get EC points to advertise on other sites or even just to socialize on its forum, but maybe you can find interesting blogs in your journey, so it's worth the trouble. :)
Do note, like it or not, Entrecard helps us build relationship with other bloggers. ^^; EC dropping system can lead to reciprocal drops which is usually lead to mutual friendship. So yes, I recommend it to be installed on your blog. Happy card dropping.