Yep Anebel, as I said earlier, a slowed walk DOES have its place. But there is no reason for the horse to come behind the leg when you slow the walk. My boys walk isn't enormous, but it is very correct, he overtracks nicely without being excessive, and it is adjustable without a tendency to go lateral.
To be in front of the leg, the walk doesn't have to be huge and fast, just reactive and adjustable.
There is definitely a balance though...
My horse tends to be behind the leg and so it is a big thing for me to keep him up infront of me, however sometimes you have to give on that in order to maintain a correct gait.
Now that he is schooling a bit from the I2 and giving some good short steps the collected walk is coming infront of my leg but for the past 2 or so years yes, the walk has been a bit behind my leg to maintain a correct rhythm, relaxation and a contact.
I don't believe that especially for a horse who has trouble staying infront of the rider that we can expect that 100% of the time. It is something to work towards BUT imo the rhythm and relaxation always come first... There is always a give and take in our training and many times I feel that riders are so focussed on getting the horse forward and they forget that there is also a slow part that needs to be addressed. We can't be running the horse's forward to get this "infront of the leg" feeling - there has to be a lot more balancing back than I personally see in the dressage ring, and especially at the lower levels and with young horses. I am finding more as I school my own horse who tends to fall behind the leg that if I slow his forward motion down enough I can actually get him more infront of the leg by his own accord than if I were to immediately do a transition to a more forward walk or trot...
As far as the adjustable part.. how adjustable?? There truly is no limit to the adjustability of the paces until they can all be performed on the spot, and even backwards as well as to the point in the extensions that the joints truly are fully extended. The training for the most part is about increasing this longitudinal ductility and simply accepting what the horse is capable of naturally as "good enough" is not, in essence, training.
What it comes down to is compressing the body by loading the hocks and raising the shoulders for true collection. When the horse is behind the leg, he tends to fall then over at the shoulder when driven forward - why not keep him in his comfort zone and bring the shoulders to meet the slower (behind the leg) haunches and when the body compresses and the hocks lower he can more easily find the collection (and thus - become infront of the leg) in his own comfort and balance - and then we can bring him into a longer stride or more forward gait. Teaching the horse to contract his abs and load the hocks is singularily the most difficult as it is not only hard in the body for them - but hard for the rider to explain.. I don't feel that with the horse driven so far forward it makes the explaining any easier. Yes forward has it's place and it is important to school the horse in working gaits, but if we leave the horse too long in the body it becomes far too difficult to teach true collection...
Just my thoughts and why I don't ride forward much - especially in the walk.