sometimes it's all or nothing

 וְאִם תֵּלְכוּ עִמִּי קֶרִי (26:21)  Rashi explains רבותינו אמרו: עראי, כמקרה שאינו אלא לפרקים, כך תלכו עראי במצותיו.  It sounds from the pasuk, writes Rav Shteinman, that doing mitzvos in a haphazard, careless way of עראי is worse than not doing them at all.  He then quotes a fascinating comment that he heard from the Brisker Rav in the name of R' Chaim.  When Eliyahu had his standoff with the nevi'ei ha'Baal, he challenged Bnei Yisrael (Melachim I 18:21)  עַד מָתַי אַתֶּם פֹּסְחִים עַל שְׁתֵּי הַסְּעִפִּים.  Choose -- either follow Hashem, or follow Baal, but you can't do both.  R' Chaim asked: if someone took a bite of treif, instead of telling them to stop, would you instead tell them that they might as well finish the whole meal?  מי שאכל שום יחזור ויאכל שום ?  Obviously you can't compare the issur of taking one bite to an entire meal.  Why then did Eliyahu tell the people that they might as well become 100% idol worshippers?  Isn't 50% observance better than nothing?

R' Chaim answered that when it comes to shemiras ha'mitzvos, every single mitzvah counts, every aveira avoided counts, even if a person cannot reach 100%.  However, when it comes to emunah, there is no such thing as 50%.  Either you believe, or you don't believe.  There is no such thing as hedging your bets.  Eliyahu was dealing with a crisis of emunah.  In that case, it's all or nothing.

וְאִם תֵּלְכוּ עִמִּי קֶרִי doesn't mean you make minyan only 50% of the time.  It means, based on this R' Chaim, that a person's emunah is at 50%, and because of that, their commitment is lacking.