Can anyone explain to my why it's so important to worry about cash flow or cash on cash ROI when you have a property with an excellent cap rate? Why should the initial returns matter so much to buy and hold investors?

Here's my concern:

If I put 10,000 into a mutual fund this month and $50 each month after that (total of $9k), in 15 years I would have about $34,000. No one I know would consider this a "bad idea." (Though, there are certainly better ways to invest your money.)

However, if I put $10k down on a $180,000 multifamily property that has even a -$5 monthly cash flow due to a 15 year fixed rate loan, the consensus on this site seems to be it's a "bad deal."

Now I agree that it's not a perfect deal, but if I can secure it with low money down and seller financing, I feel like it would be excellent to spend a couple bucks out of my own pocket each month (if I even have to, since I run my numbers so conservatively) to have a multifamily property that is entirely paid off in 15 years. The cash flow after that point would be great and the whole property (minus a few repairs) is paid for by tenants.

I have a day job that I enjoy and I have other cash flowing properties already that more than make up for one that doesn't. We're not trying to retire tomorrow.. but would maybe consider it 15 years from now.

Bonus Question:

If I were to get a 15 year loan and plan to use an exit strategy of refinancing for a 30 year at some point, what kind of deals are available to refinance? I assume I'll need equity of least 20-25%. My typical lender sent over these refinancing requirements, and I'm not sure they are typical:

1.You must have 6 months in reserves for the subject property and an additional 2 months reserves for every other property you own.

2.You can have a total of 6 properties financed.

3.On a refinance the loan-to-value must be below 75%.

4.There is a minimum loan amount of $50,000.

Let me know if I'm way off base or seem to be missing something.