You are not wrong there and I agree with you, but not in the actual context (Windows Defender).
Nowadays almost all browsers uses Google Safe Browsing API (refer to
@Evjl's Rain tests), so holistically and considering Smart Screen + the free Windows Defender extension, the user isnt missing a web filter.
Windows Defender has a bonus that it doesnt break SSL and has minimal impact on browsing speed.
About the behavior blocker, I disagree there, it isnt the most important layer, but the most powerful one; the most important is always the good and old well-crafted signature detection (contrary to popular opinion, it is hard to get a zero day malware infection).