I'm not pressuring one way or the other, but I took three screen shots with my UI layout and 'Shift+F' (focus target bar) so you could see what I do and maybe it'll help. I also watch casting bars and moved most things from the 'top' of my screen to directly above my action bar, near my party list as I main WHM and spend a lot of time looking to my Party list. I moved my party list to the Right hand side for example because that was a more fluid motion for me than moving up and left to target ppl. (Plus the party list use to be on the right in 1.0/FFXI and I'm probably a little use to it).
Anyway, in both screen caps you can see the enemy's 'cast' on both the focus target bar. This is an example of 'healer typical party member main targeting and what we see', as my chocobo is my actual target. As you can see, the <tt> bar of my target is the mob and does NOT show the Mob's cast...but 'Focus Target' does.
In the picture below, you can see the Enemy's cast on the enemy target bar when i have the enemy directly targeted. But you are correct, unless you do a lot of macros, it isn't easy to see an enemy's casting bar as healer since your target is typically the party members (you would have to target the enemy and heal via <tt> macros...but that gets a little odd).
I also like doing 'Focus Target' on other party members when I have to be looking out for specific debuffs etc. (Iffy Extreme and my SCH partner for example...as that way not only can I see the healer debuff, but I can see his exact timer to know when I need to swap positions all while being able to actually target whoever I need to cure/dd etc).
I made sure to go out and take these screens with my chocobo since that way you could see party list placement/what happens when a party member is targeted instead of a boss.
In general though, I rely on battle cues and only use Focus Target for specific things.
PS: I was just using efts/chocobo to give a general idea of my layout...hence no protect real spells etc. (IE don't judge what I was doing I was more so focused on catching the mob during bog bomb than doing real dd/heals).