MN - Looking at 4 plex, current owner does month to month only

[Frank Cannata]

I went and looked at a 4plex this weekend, the owner happened to be there doing some work on the property so we got to talking with him which was awesome, he has owned and managed the place himself for 13 years. 

One thing that had me confused though, he said the previous owner he bought the place from told him to only do Month-to-Month leases with tennants so that "he can avoid paying the $500 to evict them". He said he just "has to make sure he sends them a 30 day notice and he is good." He has been doing this for 13 years. Currently has one of the units rented out 4.5 years, the other unit 2.5 years, then the 2 other units have people coming in right now. 

This is in Minnesota. 

does anyone know what the benefit would be to keeping it month to month? Is he correct? or has he just been extremely lucky for 13 years? seems like everything i read says i want to get my tenants on a year lease. 

thanks in advance for the help!