If you use the preamp in your amp to get gain, the effects loop becomes more important since effects like delay or chorus sound best after distortion. If you use the amp for clean or light breakup with pedals to give you overdrive, you can put the chorus and delay after your overdrive pedal and the effects loop isn't as important. Its also important to realize that anything in an effects loops will affect your tone. There isn't a right or wrong just recommendations. IMO, I don't need an effects loops since I prefer a clean or light breakup amp with pedals. If you use a high gain amp such as a mesa, you might get better results using chorus, delay, etc in the loop.
I have no need or care for a loop either. That being said, I have messed with most of my amps over the years running modulation effects in the loop versus in front just to hear the difference. Some of those amps had fantastic sounding loops.
But yeah, none of my current amps have loops and if they did I would not use them live.
unfortunately it is essential as i tend to use delay for many different applications and have never found a distortion/od pedal that is authentic enough for me... i won't consider an amp if it is loopless.
I can only think of 2 reasons why a loop is ever necessary (term used loosely), and both of them start with the premise that you are using the amp's overdrive (whether it's a master-volume amp or not). If you're using the amp's own gain, then you might want a loop if:
1. You like to use delay or delay-based effects (chorus, flanger) and don't like the cluttered, lo-fi sound you get when you run a delay into a distorted amp.
2. You want to be able to cleanly boost your volume (for solos, etc) and your amp doesn't have a boost feature built-in.
Loops are needed for ambiance effects (delay &/or reverb). Not necessary for boost/overdrive, wah, or modulation effects (chorus, phaser, flanger).
If you're going to be using a dirt box to generate your distortion tones, (if you can put the dirt box in line before the delay/reverb effect) you don't need a loop.
On the other hand, if you don't use delay or reverb, you have no use for a loop. I do use reverb and/or delay, so I must have a loop. I wouldn't buy an amp that didn't have that feature. But that's just me.
Effects loops are a must for me , i can't stand any time based effects in front of a distorted amp.
That being said , i also learned about what type of effects loops i like better than others. There is a big difference in a series verses a parallel loop design , and one thats set for -10db effects or +4db line level effects.
I've been running Soldano amps the last couple years and they have a set +4db series loop design. If you like analog floor pedals , most won't work since they are mostly -10db input rating. There are a few that can handle hotter +4db signals without overloading the pedal but limited in choice.
So people that need loops and are looking for a new amp or a loop mod done to their existing amps , make sure you know what the loop is designed for before making a decision.