Do not spend time trying to turn an undesirable suspect into a client; spend time FINDING those who need your expertise and product, and are willing to PAY your desired price5.

The "undesirable for incorporation" comment in the wikipedia article is likely to change in the next decade or so IMO.  

When I hear about a situation or action which I find undesirable I think Yuck.

It's those areas that will have lots of cash buyers.That being said, unless you work an area that is really remote, really undesirable or really high end, you are not going have a problem finding cash buyers in this market.

Dont trust anyone or you will end up holding a undesirable property.

Evicted undesirables in one side of the unit and re-rented.    

With respect to the law of building codes, permitting and contractors, I have to be frank--in Ohio, it is nothing more than a hidden tax, a protective tariff, and a way for the municipality to veto any development or renovation that its architectural review board subjectively considers "undesirable."

Some related reading for you, since you seem to not like research (an undesirable attribute in an agent, BTW):http://www.biggerpockets.com/forums/21/topics/170-how-to-find-a-good-realtor-http://www.biggerpockets.com/forums/21/topics/74296-what-defines-a-good-re-agenthttp://www.biggerpockets.com/forums/21/topics/69232-what-do-you-actually-look-for-when-selecting-an-agenthttp://www.biggerpockets.com/forums/311/topics/70483-how-to-find-a-good-re-agenthttp://www.biggerpockets.com/forums/21/topics/69807-value-of-using-an-real-estate-agent-in-the-deal

Many homes that are bought by investors would not qualify for traditional financing anyway, so they would either sit vacant which attracts vandals, druggies, and other undesirables, OR an investor rehabs and resells.

Those 20% are undesirable areas.