Making future alterations easier is certainly true. You have no "center stripe" in the rear to worry about, as John points out. Also, it makes it easier if you have to cut out any pleats to make the kilt smaller. A few times I've had to remove several inches of pleats to size kilts down, and the fact that they were pleated to stripe meant you could never tell by looking at the kilt. Had these kilts been pleated to sett you would notice a significant portion of the sett pattern missing.

However, I would not say it is necessarily true that pleating to the stripe uses any less cloth than pleating to the sett. Let's say the set size is 7". If you are pleating to the stripe, you are going from one stripe in the tartan to the next occurrence of that stripe and so the pleat is going to be 7" over. If you are pleating to the sett and your pleats are 1/2" wide, then your next pleat would either be 7.5" over or 6.5" over to get to the point in the pattern you need. So pleating to the sett may use slightly more cloth, or it may use slightly less.

Effectively, it's very little difference. If you are ordering an 8 yard kilt, your kilt maker will request 4 yards double width cloth from the mill, regardless of whether your kilt is being made pleated to stripe or sett.