If it's a season ticket holder of if it's a single travel reservation I still don't see the problem if the TOC makes it totally clear when the seat is reserved so other passengers can easily see when it's not taken up.
Because if a seat is reserved then many people will not use it, for a variety of reasons. Not everyone will go inspecting every reservation label as you do in the hope of finding a seat. Even if they do that, it dramatically slows down boarding times at busy stations. I will give you a personal example to illustrate one of the reasons for that.
One day last August, I boarded an HST at Newton Abbot with a friend, both without reservations. The unreserved coach was full, so we settled on two seats in a fully-reserved Coach B, where there were plenty of seats reserved from Plymouth but not taken up.
The reservation holders then turned up at Castle Cary, almost two hours after Plymouth, and demanded that we vacate these seats. By then there were no seats left at all on the train (we confirmed this as we walked through Standard Class accommodation in the aftermath) and the vestibules were virtually full so we refused. The reservation holders of the six seats around us (all from Plymouth) never turned up and we were not prepared to play musical chairs with untaken reservations. To compound matters, the guard then demanded that we move, totally contradicting the advice given on twitter by customer service.
We moved at the end, as I was not really in the mood for a "fight". With no spare seat in sight as we walked through the train, we got off at Westbury and travelled via Bath.
There is no consistent approach within the industry as to what happens in these circumstances (even within the same company as my example illustrates). I have evidence that Virgin agree with the interpretation that you do not need to move should the reservation holder turn up several stops down the line. I also have evidence that XC claim you need to vacate these seats whenever demanded but you could not be forced to do so. Then another TOC (name slipped my mind right now, but possibly NXEA as it were) claimed that you must move whenever demanded. In reality, what many people will do is simply occupying the vestibule areas when they see coaches almost fully reserved even if plenty of them go unclaimed, thereby clogging up movements for others.
If reservations are charged for, you can reduce the number of unnecessary reservations and minimise those problems.
I am not going to get into a debate as to whether the seat should be given up on demand as we have been through that many times in other threads. I am only expressing my view here to illustrate my point. Anyone wishing to reopen that debate will have to do so in a separate thread.
(To anyone wondering, we had the last laugh at the end on that day, as that train got caught behind a train which failed, sent on a diversion, and then in the midst of signal problems at Southall on the fast line, while we sped past on the slow and arrived 20 minutes earlier at Paddington.)