Should I use my 50% rule reserve money to invest in a security?

[Samuel Hall]

Hey folks, NewB to the RE game here. I am currently studying for my RE License in Colorado and want to invest in rental properties. I have learned about the 50% rule (explained here https://www.biggerpockets.com/blogs/4454/32123-wha...) 

and was wondering if any seasoned investors saw a problem with using the saved funds in a highly liquid investment vehicle.

for example: 

I save 50% of my rental income in a separate account to compensate for maintenance and vacancies, and use that money to purchase dividend bearing stock. When a major expense occurs, I can easily sell the security and have cash to pay for the expense. 

Has anyone tried this strategy? how has it worked for you?

Thanks for the input, I hope to (L)earn much from BP.

Samuel Hall