I am willing to concede that it isn't alway about the right tool for the job.
We've got lots of different configurations at our disposal: single tank, backmount, sidemount, multiple stages, rebreather, sIdemount rebreather. WIth training and practice we can move between them.
Then we've all got that one zealous friend that absolutely insists "This one configuration is the absolute best for absolutely everything absolutely no matter what!!!!!!!"
And as zealots frequently are, they're usually wrong.
There is no reason to introduce the multitude of additional complexity of a sidemount rebreather when the dive is a 30 foot reef dive off a six-pack. There are some caves where you can't fit in doubles... period. It's not even a question of debate; you can't fit through a 15" clearance with a 22" profile.
Sure, most of the time, certainly on more aggressive dives, it is about the right tool for the job. But all of the time is about practice.
Do you have it? Can you use this tool or that capably? Have you been diving this or that configuration recently enough that you have the muscle memory to appropriately respond to an emergency? Can you rescue yourself or a teammate? If the answer is, "no" then perhaps it's time to dive that configuration for a while. (Not once and say, "There we go... refreshed!" A while.)
Do you not have the opportunity to dive as often as you like? Do you, then, wisely recognize it truly is safest you stick to a single configuration to maintain absolute proficiency with the rig in which you are most comfortable?
Or for those of us lucky enough to dive as much as we do like: on less aggressive dives, are you just having fun?
I switch around a lot. Recently I've been diving my rebreather a bunch, in fairly "benign" environments. Doing so has the nice effect of proficiency maintenance... but mostly I've been doing so because it's fun. It's what has grabbed me and made me smile most over the last few weeks.
And that truly is what it's all about, isn't it? Even for all the talk about "the right tool for the job"... for most of us it isn't a job. It's just some dumb shit we do because we enjoy it.
There is no single right tool. Go ahead, dive what you like, enjoy it. Safely.
All that said: doubles are still the right answer for 90% of technical dives and I will still make fun of anyone bringing sidemount on a boat.