I love my sidereins. But you need to be using them for the right reason. People that verbalise that they are using them to 'get my horse's head down' are what gives side reins a bad rep. They are not designed to be used as shortcuts. They are designed to assist in giving the horse a steady contact to work onto on the lunge, to help establish the idea of working into a steady contact from the hind quarters, which in turn engages the hind legs and back.
Lunging with nothing in my opinion, does nothing more than allow the horse to burn off some energy and maybe listen to voice aids.
I know when I lunge, my aim is always to have my horse working through from his hind legs, over his back and into the contact, not backing off and not lugging. Without an outside rein at least, the energy you create at the hind legs travels up over the back but then straight out the outside or inside shoulder, generating a horse that ducks out or drops in on the circle, and after a long period of time, you will simply be building up incorrect and uneven muscling, making work under saddle a challenge.
Side reins are the ideal 'hands'. You want your hands to have some give, but keep still and quiet to encourage the horse to take the contact. Side reins to just that, hence why I like the elasticised ones. They have that slight give in them, but remain steady and constant, which, if you are doing your job on the hind end well enough, will give the horse comfort if they take the contact.
The use of side reins should be controlled by a horse person who knows exactly what they want to achieve with them, how they work and why they are using them. Having side reins attached so they are loose and swinging around is 100% pointless. They are there to give the horse a contact, so use them for that purpose.
Then there is the opposite dilema of adjusting them too tightly (most often done by the folk who follow the rule of using gadgets to jam the horse's head in because it 'looks pretty'). This will simply create either a horse that backs off the contact to escape the excess pressure, or a horse that leans heavily onto the bit.
You must find a happy medium, and this can be longer or shorted depending on the length of neck, the level of work at which the horse is peforming, and it's experience in side reins previously.
Obviously the younger less experience horse will need the reins longer than an older horse working in collection. The most important aspect of successful use of side reins is that you work the hind end effectively. The hind legs MUST be swinging under the horse taking it's weight, and the back must begin to loosen and swing in accordance to the hind legs.
I always keep my side reins at the same length. Shortening the inside rein only encourages over flexing to the inside, and gives the perfect oppertunity for the horse to duck out with the outside shoulder.
When you ride, you want your reins to be even, so it is the same with sidereins. The inside bend will come once the hind legs are working, the back is swinging and the horse is taking the contact. It does NOT need to be encouraged by shortening the inside rein to pull the head to the inside.