Cue extensions are supposed to be used only temporarily on a per shot basis. This is because they add weight, change cue balance, and affect the overall cue hit.
Wow! The above sounds like it must be in the rule book somewhere.
I started playing with a Balance-Rite a couple of years ago and I liked it
a lot. I think it's a problem to add an extension to a cue for the occasional long reach shot. These shots often require a good feel for the speed required and adding several ounces to your cue (for just that one shot) makes the required feel more difficult to realize. Those shots seem a lot easier if the weight doesn't change.
In my experience, I'd say that these long reach shots come up once every rack or two. That's pretty often.
So what's the down side? The extra weight added by the extension is a non-issue. I think anyone can get used to it in very short time - minutes. There are a number of places that don't have enough room for extra long cues. Because of these, I often seek out a table with a lot of clearance all around.
The balance issue deserves a little more discussion. For me, the only difference between two otherwise identical cues, one that is forward balanced and one that is rear balanced occurs when I cue at the near edge of the table with an open bridge. In that circumstance, there is a little less weight on my bridge when using the rear weighted cue. Other than that, I can't tell any difference between the two cues. I expect that a lot of players think that's not the case - and I'd like to hear their version.
For the last six months or so, I've traded up from the Balance-Right to an extension that fastens at the butt. It adds eleven inches to the cue instead of the Balance-Rite's four and a half. That makes my 59" cue a full 70" and I'm afraid that even longer is even better.