An individual will only be engaged in off-the-job training if they are an apprentice, and they should only be an apprentice if the employer has agreed that they require significant new learning (which meets the minimum threshold) to be able to perform their job effectively. If the individual does not require this level of training and can perform their job adequately without it, then they should not be engaged on an apprenticeship.
For those that are engaged on an Apprenticeship there may be an initial loss of productivity, due to the time the apprentice is engaged in training, but in the long term, the new skills that the person brings back to the workplace, which makes them fully occupationally competent, should compensate for this. An apprenticeship is one of many programmes on the market and a different programme may be a better fit for an individual or their particular employment circumstances, e.g. those that require a smaller amount of training or those where the employer cannot commit to the amount of training time to be delivered within working hours.