Talking about "pace" does not necessarily mean that's what you think the player will end up with, but it does have value as it can be used as a tool to conceptualize how well a player is doing at a given point in a season. For example, if I told you a player has 15 HRs at some point in a season, if might not mean much, but people do know that hitting 45 HRs over a full season is pretty darn good, so if a player has 15 HRs after a third of a season, the "on pace for 45" just shows the pro-rated value of 45. If a player had 15 HRs after 75% of the season, he'd be on pace for 20. It's all relative, and so I do think using the "on pace for" phrase does have value. It's not absolutely worthless.
We all know a guy who hits 3 Hrs in the first two games of a season might be "on pace" to hit 400HRs, so in the limited sense like this, the statement is useless, but once you get closet o half a season, it does have conceptual value, but not necessarily projecting value.