People often evaluate training simply in terms of 'does it work'? Sounds reasonable.
If they mean 'does it change the specific behaviour we are focussing on, in the way that we want' then it is definitely not enough.
Every interaction we have with a horse may change lots of things other than the specific behaviour - for example they also learn about the individual trainer, may generalise this learning to all people, may come to feel more relaxed or more worried in certain environments, with certain equipment and so on - all as a result of one simple piece of training.
Just rereading a paper, 'Reinforcement as a mediator of the perception of humans by horses (Equus caballus)', Carol Sankey et al. To quote from the abstract:
'In this study, we tested the hypothesis that the use of positive or negative reinforcement in horse training may have consequences on the animals’ perception of humans, as a positive, negative or neutral element (in their environment). Two groups of ponies were trained to walk backwards in response to a vocal order using either positive or negative reinforcement. Heart rate monitors and behavioural observations were used to assess the animals’ perception of humans on the short (just after training) and long (5 months later) terms. The results showed that the type of reinforcement had a major effect on the subsequent animals’ perception of familiar and unfamiliar humans. Negative reinforcement was rapidly associated with an increased emotional state, as revealed by heart rate measurements and behavioural observations (head movements and ears laid back position). Its use led the ponies to seek less contact with humans. On the contrary, ponies trained with positive reinforcement showed an increased interest in humans and sought contact after training. This is especially remarkable as it was reached in a maximum of 5 sessions of 1 to 3 min (i.e. 5 to 15 min) and had lasting effects (visible after 5 months). Even learning was positively inﬂuenced by positive reinforcement. Overall, horses seem capable of associating humans to particular experiences and display extended long-term memory abilities.'
Common sense, you might say - if we do nice things with our horse they want to be with us more, if we train them by doing things they find unpleasant, they want to be with us less.
This is only a little part of the picture of 'good' training - for example simply throwing large amounts of food at your horse for every good thing he does is probably not the answer either! But it is an important element that is often overlooked, by scientists and trainers alike, so it's great to see some research in this area.