In theory regulars are either fit or under treatment/review, as they are all cared for by the military medical services, and thus the military are aware of those unfit to deploy. The truth, of course, is that it's actually a slightly blurred picture because the DMS was decimated and does not offer the level of service that it used to. Too many CMPs and too many civilian nurses in med centres; too much NHS influence in the disaster which is the MDHU setup; too little pressure on the government by senior officers.
In the past there was sufficient flexibility in the system to allow for a percentage of soldiers to be downgraded - few units now have the manning for this.
Some units do have any downgraded soldiers examined to see if they can be upgraded sooner.
Apart from the reasons already mentioned, it makes sense for TA pers to be medically examined is that there are a few who will try to get out of being mobilised by shopping around civ doctors to get the opinion they want.
The MAOCH system is well overdue for review; it hasn't changed since before the military hospitals closed and simply can't cope with the numbers of regulars, let alone TA soldiers.