IRULE for SNI is necessary

You can use Server Name Indication directly within the VIP without an iRule:

 

https://support.f5.com/kb/en-us/solutions/public/13000/400/sol13452.html?sr=56753139

 

And you can do pool selection based on SNI within CPM (LTM policies). With at least 11.6x, you don't need any iRules.

 

SNI-based cert selection and pool assignment are generally two different things, so if you can do SNI at the VIP, and you are offloading SSL at the VIP, then pool selection can be based on HTTP events:

when HTTP_REQUEST {    
    switch [string tolower [HTTP::host]] {        
        "foo.site.com" { pool foo_pool }            
        "bar.site.com" { pool bar_pool }            
        "blah.site.com" { pool blah_pool }
    }
}