I am guilty of falling into this trap on half-par holes, and Dan's statement is absolutely correct. You play to get the ball down in as few shots as possible based on where you are. If you're 200 out with water next to the green, how you play it should be dictated by your ability, not whether you are trying to reach the green in regulation or one below regulation. But we all know that the par does affect how people will play that shot. I'd imagine people would be quicker to lay-up on the Par 5 than on the Par 4.
Having acknowledged that I understand the theoretically correct answer, I still think the assigned Par shouldn't be blatantly inconsistent with the demands or risks presented. If it is inconsistent either way (too low or high), it will never be a great hole.
Consider a 480 Yard Par 4/5 with no real demand other than length. There is no hazard near the green to make "going for it" a real risk/reward scenario. In this case, it would either be an uninteresting, pushover Par 5 or a boring slog of a Par 4 (this would be the true 4.5 Par holes). Changing the assigned par isn't going to save it either way.
Now, if you take the same hole and add real dangerous hazard near a tricky green, you get an element of risk/reward going for this one in two. Now, it becomes more interesting as a Par 5, but an even tougher slog as a Par 4. In essence, now you have a Par 4.75 hole. Even though the shots are the same, trying to pass this hole off as a Par 4 just feels wrong and inconsistent.
At Augusta’s 13th & 15th, I think Jones & MacKenzie knew they had Par 4.75’s but felt the potential for a big number made it obvious that they had to be designated as a Par 5. I can’t ever imagine these holes being viewed the same way if they had been designated as Par 4s.
For a while, technology advances had the 15th bordering the Par 4.5 territory and it was losing its appeal (whether they had changed it to a Par 4 or not). At least Augusta didn’t take the USGA route and just reduce Par by one and call it a day. They brought it back up to a 4.75 and maintained the original intent. Unfortunately, looking at the set-up for Congressional, it appears that the USGA is content to just adopt the uninteresting, long slog approach.