Should dbo schema be avoided?
It may be a good practice because when you have other users using the database you want to be able to limit their access with schemas. For example in a database you have the following tables.

HR.Payhist
HR.Payscale
HR.Jobdesc
IT.username
IT.useraccesslevel
ENG.jobsite
ENG.trainings


As the HR director I am able to access anything in the HR schema, as the IT director I can see employees usernames and access levels. The Engineering department can see what job sites are active, etc. If dbo was the set schema for all the tables I would have a harder time segmenting out my data and providing access roles.

The idea, I believe, in SQL Server is to offer a product that can be access and queried by different departments. In reality only DBAs/DBDevs really access the database and it typically only stores application data.

It also helps with readability and manageability. At first blush I can easily identify what table holds what data and how the data is separated.

Personally I prefer defining schemas as a general practice. Remember schema is greek for plan, having a laid out schema structure helps you to plan and identify data.