Let me give my reasons.
1. Possibility of abuse. A tired batsman could 'fabricate' an injury and ask for a runner and may be permitted one.
2. At a vital stage of a game, permitting an 'agency runner' may be crucial. The extra runs that a fit runner can 'score' for the unfit batsman (unfit to score by running) could be the difference between victory or defeat.
3. Relieving the batsman of his job of running will keep him fitter throughout his innings.
4. The lack of a counterpart facility for the bowler. A batsman not fully fit may still bat and score with the help of an agency runner. A bowler unfit to field will have to leave off bowling, or bowl and field. Discrimination, because fielding will tire the bowler, while the batsman rests employing a third party to do part of his job of scoring - which comprises both batting and running. I would suggest one of 02 alternatives, to reduce the disparity:
1. Withdraw the facility altogether. If a batsman cannot run and is unfit he should retire, and return later at the fall of a wicket.
2. Permit the facility for a limited period, marked either by a time period (say 15-20 minutes) or by a number of overs (say 5-7 overs). ``Sportex'' Colombo