Why are PMIs so bad?

[Travis Washington]

I've noticed many investors caution (or pretty much flat out say avoid PMIs on loans) but why?  I am analyzing the scenario with a deal of mine.  Below are the numbers:

Property Price: $47.5K
Projected Rent: $675

At 20% down:
Down Payment: $9,500
Rate: 4.85%
Payment: $320/month

…Now, At 15% down (and PMI):
Down Payment: $7,125
Rate: 5%
Payment: $356/month
PMI: $9.08/month (~$109/year)
Total Monthly Payment (above two together): $366/month

My evaluation on this is I only pay $25/month ($300/yr) more to save and hold $2,375 for securities and future investments.  At that $300/yr rate, it would take 7.9 years to reach the $2,375 that I would've held up front and I would use that cash to assist in acquiring another property. In addition, a $25/month addition doesn't seem like that big of a damaging hit to my cash flow.

Now, maybe I'm missing something (and that's why I bring it up here), but the 15% option seems like the choice here.  Less skin in the deal and more opportunity to spread my investment.  Thoughts, BP professionals?

In addition, if I wanted to eliminate the PMI from the mortgage in the future, at what point am I available to do this?

Also, does that effect my chances of purchasing a personal home or what I'll have to put down (will be using the VA Loan)?