I cut off loops whether on line or leader, but I certainly understand the usefulness of them in quickly switching leaders and or tippet. I make too many casting mistakes and tailing looping a dry into a leader loop is a mess that does not happen if there is no loop there to start with. Same with dry/dropper fishing. This is how I do probably 90 percent of my fishing if not more.
On the other hand, if I grudgingly resigned myself to mainly nymphing for the day becuause of the conditions, then I would likely use one for the leader butt connection to the leader along with a tippet ring for the flexibility it allows to change sections quickly, escpecially if also using shot which dings up mono and needs replaced regularly.
Loops are ok for this because in all likelihood the delivery will be more open and hence avoid the loops getting snagged with flies during the cast. But I am using the same fly line so there is no getting around learning the nail knot for that first connection from line to leader butt, which then gets a perfection loop at the end.
I tie my nail knots free-hand and once learned is very easy. I have used knot tools but my experience with them was that they do not allow the exact placement of wraps without springing out of place when you remove the tool when using stiff, bulky mono. Can't rule out my own fumbling nature in that so it may not be the tool itself just my lack of ability to use one.