I heard someone say they made a 50/50 blend of soilless potting mix and regular garden soil for a container planting. Why would this be done and for what types of plants?

A 50/50 blend of soilless potting mix and regular garden soil is fine as long as you can control the amount of water the container will get. Garden soil will retain moisture for longer period of time which can be helpful with smaller containers or hard-to-water areas.

The negative is that we cannot control the weather, so during the summer when we might get a few rainy days in a row, you could easily end up with root rot.

A straight soilless mix does dry out faster, therefore more frequent watering is needed, but you have a far less chance of overwatering and your plants will root out much faster.