It is quite common to not have the slave in read_only mode..
in fact, if you do not set it yourself, then it never is enabled..
A good question, is why you would actually want to set it...???
Are you trying to protect mistakes? or errors made my admin users??? or the application?
Considering MANY admins are using MySQL as root anyways, where is the protection here??
You cant exactly protect your data as you would imagine ( as everything replicated into the slave anyway )
You do know that any user with super privileges and the replication thread do not abide by the read_only??
A strong structured approach to users and privileges is the correct way to deal with replication issues, not a read_only configuration on the slave
In my honest opinion, it actually does not protect very much...or useful for anything besides stopping a few users not write.. ( which on production systems - should never happen anyways )
As an example, In Mysql-Multi-Master ( read_only is set by the scripts )
This in theory is only to simply decide which server is the "True Master" in the master-master configuration..
All in all.. Forget about read_only ( its really unneccesary, and probably wont protect you from what you imagine it will )