In my UI I try to use as much of WoW’s default frames that I can. I do alter some by scaling or repositioning. The reasoning for this is simple: major patches don’t break them nor are they “missing” certain things that blizzard expects you to have.
I remember back in ulduar when some people couldn’t see the energy of their vehicles on the first boss nor did they have an updated mod that changed into the vehicle action bar when they hopped on. We wasted time waiting to have them download the newest version which didn’t fix the issue because they needed to download a beta or alpha version or it wasn’t available at all. Malygos targeting of the drakes proved strange for some raid frame addOns as well. Anyone with default frames even back then had no issue. Blizzard makes sure that whatever targeting you need during a vehicle fight works.
I was using a player and target frame add-on to level in jade forest when I came across a quest that had a balance bar. Unfortunately I found the quest impossible to do without the extra bar. I turned off the add-on and one-shot the quest. This reminded me of ulduar. I had the newest version available, yet I didn’t have an additional module for it just to display this one bar. Some buffs/debuffs don’t show up on raid frames unless specifically added. Some weird mechanics like absorb shields are strange on some encounters.
Completely broken mods
If mods break you can just turn them off and still be 100%? I don’t think so. When you keybind using add-ons or set up additional action bars those all can disappear when you turn them off or they break. Now we’re talking wasted time while you quickly redo or keybind the default action bars again. If you’re one who logs on just in the nick of time for raids you’re either sitting out or everyone’s waiting for you. That’s just for action bars. Anything else you’re used to being in a certain place may not be there or be presented in a different fashion. Try making cool macros in some add-ons and have the add-on break.
Don’t forget that as World of Warcraft ages some authors leave the game. Others pick up the addOn and “maintain” it as best they can. Some only sub for new content. That “fix” for whatever you need might be a while before it gets released. The default frames are always ready.
Why the default frames?
Blizzard frames are pretty good anymore. At one time I wouldn’t have said that. What I do use occasionally are add-ons that give benefit to the Blizzard frames or ones that do things the default frames won’t do. The worse thing you can do is have an important role on raid night and waste other people’s time by your add-on choices. You get frustrated toward the add-ons, everyone else is frustrated for you not being ready, and all that happens is wasted time. There’s no wasted time when my UI doesn’t change much when add-ons break.