I have been infected by malware (with widows defender active and updated, BTW) on an unattended computer. Just parked on a website, and a compromised ad-server rooted my machine. Of course, Microsoft wasn't interested in the forensics, either.
You might say that "leaving a browser open is questionable", but I think not.
It is not a matter of "if" you will be compromised, it is a matter of when. With windows defender, the odds go WAY up. Did you even look at the AV comparatives reports? If you are aware of the 83% effectiveness (worse that all other solutions) and poor performance of windows defender, then that's your decision. But, if you are actually unaware of the quantitative differences between the many AV solutions, then I don't know what else to say.