ADUs will substantially increase the value of a property and provide an owner an alternative stream of cash flow.

If applied farther up stream to the seller and the buyer, your theory still is to generic to be a valid point. 

and when most folks are making less than 10k a month.. and tax's are taken out etc.. the idea of getting a 25k check all at once is just too appealing. then just do it every month and there you go.Same with those that think they are going to retire on 200 a month cash flow homes.. while they are a nice addition to the income stream it takes a boat load of them to really make any significant income to replace your 9 to 5.. unless you already are starting with cash and can buy a couple million worth free and clear.  

The issue I'm facing is that I'm beginning to look at properties in the Midwest with a low acquisition cost and rental stream (relative, of course).

Read the Ultimate Beginners Guide linked at the top.It's pretty similar to analyzing a SFR, just with more income streams and potentially slightly different expenses.There's also several good books out there on multifamily analysis, e.g.

A two,three, or four family home, or another property that allowed for one or more income streams.

The reality is that an investor will deal with both over the life of an investment and having multiple doors allows, in theory, for a more consistent income stream to cover costs.  

@Marissa Myers I put a Roku device or a streaming BluRay player with all the HDTVs in the living room. 

My mortgage payment on the property is $21,500 a year.Guest pay all state and resort taxes.Guest paywall cleaning feesExtra money stream for us.Security Waiver Fee (self insured)Pet FeeCleaning Fee, we charge $150 per visit, cleaner charges us $85 to $100Most Vacation rental areas have a high season with 4 months of high season.Usually with 4 months you can pay mortgage, insurance and taxes for the yearA couple of good books areThe Vacation Rental Goldmine  by Debusk ChrisHow to Rent Vacation Properties by Owner by HRIB-Karpinski-Christine

But, I don't think of it as buying RE, I'm buying an income stream.