I think it's complex, and while the results of regressing behaviour could be negative or positive depending on the child, I don't think regressing is good or bad in and of itself.
Two of my children regress when they're particularly anxious and stressed, DD2 especially, sometimes very dramatically. I adopted her 10 years ago, and it still happens, and happened with her FC's before me.
On the other hand, sometimes her regressing isn't prompted by a particular stressor, but almost the opposite - moments where she's feeling a bit more secure - and these times have lead to us doing very positive things together and developping our relationship. I've definitely babied her a lot and treated her as a much younger child and it's absolutely been a positive experience for us.
I can tell the difference because if she gets really stressed and then behaves younger, she tends to be very 'push-pull', shoving me away despite acting like she does want physical affection. Or she'll accept a cuddle, wrap her arms roung me and immediately pinch my back hard etc. But when she's feeling more secure, she'll happily and calmly cuddle for a lot longer, or be rocked back and forth on my rocking chair etc, but still be behaving a lot younger than normal.
But either way, I take the tack that however young she behaves, is how young she probably needs to be parented as right now. If she's behaving as a toddler, whether because she's incredibly anxious and stressed, or feeling more secure but craving it for another reason, then she needs nurture and physical affection and supervision etc, like a toddler. (and also activities aimed at younger children, or earlier bedtime etc).
I have to admit I don't really like the phrase 'filling in the gaps' because it makes it sound as if it's all made up for now, there are no gaps. But in reality you can't actually 'do it over'. What's missed can't be made up for or 'filled in' by creating new experiences of this nurturing later on. The way i see it is that the gaps are still there, but hopefully we are creating an environment where new connections can be made in the brain which will help our children in their development, so they can live with these gaps without it affecting their day to day life to the same degree it might have done without this nurturing environment.