Just a few thoughts....

There is no chapter and verse in the Scriptures that states we must become card carrying members of a local body of believers. However, there are valid reasons to become a member besides having the privilege of voting. By becoming a member of a local body we are aligning ourselves with that body and saying that we are in agreement with their doctrine and statement of faith. We are choosing that group of believers to serve along side of for mutual edification, encouragement and to use our spiritual gifts for the glory of God.

Hebrews 10:25
Let us not give up meeting together, as some are in the habit of doing, but let us encourage one another—and all the more as you see the Day approaching.

1 Thessalonians 5:11
Therefore encourage one another and build each other up, just as in fact you are doing.

Being a member also recognizes that one is under the authority of the elders of that body. This is especially important if discipline issues arise.

Hebrews 13:17
Obey your leaders and submit to their authority. They keep watch over you as men who must give an account. Obey them so that their work will be a joy, not a burden, for that would be of no advantage to you.

1 Thessalonians 5:12
Now we ask you, brothers, to respect those who work hard among you, who are over you in the Lord and who admonish you.

A local body of Christians becomes your spiritual family. A true benefit to one whose natural family is unregenerate. There they can find the guidance, teaching, love and support of brothers and sisters in Christ.