Personal Line of Credit - Use it or leave it alone?

[Thomas C.]

Folks, I have a $30k personal line of credit (6% variable APR) and am wondering if I should use it for anything or just let it sit there and look like I use 0% of available credit. I've had varying responses from colleagues so I'm reaching out to the experts for advice.

My first instinct is to pay off my mortgages early using the conventional method ($300/month extra principal payments). This method obviously requires me to have positive cash flow (which I do). But there are a lot of people talking about tricks like using a line of credit to pay chunks off mortgage and then use cash flow to pay off LOC. Here's what I have calculated:

1) Total interest on rental property mortgage loan will be $104k for 30 yrs. If I apply the $30k LOC to the mortgage principal, the total interest reduces by $63k to $41k total and the mortgage is paid off in 17 years. 

2) If it takes me 10 years to pay back the LOC, the interest paid on the LOC will be $10k. 

3) If I just make an extra $300/month principal payment, the total interest on the mortgage reduces to $47k. However, it's paid off in 15 years and I've paid nothing extra in interest on the LOC!

So in my mind, just paying extra principal beats using the LOC since I will save $4k by not using it ($10k in LOC interest minus the $6k difference in mortgage interest saved). But am I missing something? Why does this strategy seem to be popular?  

I still have 15 days to close the LOC so I'm trying to make a decision before I pay anything for using it. Thanks in advance for any advice!