so you're saying a Sergeant that preforms as a Section Sergeant can use the (P) on the signature block when they are pulling that duty, or a SSG in a Platoon Sergeant position may use this as well?
That is not what I am saying at all. I am saying that there are times and situations where the fact that someone is a (P) is necessary. I already used duty as an example. How many times have you seen certain SPC pull CQ and not others, or certain SGT pull staff duty and not others. It is because they are acting in the capacity of the next rank because they are promotable. Another really good example is Equal Opportunity Leader; to attend the class the Army says you must be an E5(P) or higher. So when I was an E5(P) the paperwork I sent required that I was addressed as such.
I am not saying that anyone should answer the phone or refer to themselves as SGT promotable Snuffy. I am saying that in some cases the (P) is needed and appropriate.
When a LTC(P) is filling a COL billet, he is signing official documents that require the rank of an O6. It benefits the Army. When an COL(P) is filling an O7 billet, his (P) signature gives the required weight to benefit the Army. Even when a promotable E8 is filling the billet of an E9, it benefits the Army.
Other than that, lower enlisted or company grade officers have no real need to add the (P) to the signature other than to "benefit" their egos. I know that's brash, but it's the cold hard truth.
For instance, one of my peers was a 1SG, and he was promoted into a CSM billet. He was the selection, but the paperwork went back and forth because a period was misplaced, or the wrong serif was used on a font. But he was in the position and his troops called him CSM, so he was 1SG(P).
Here is not that big a deal. Perhaps people would think that if they were on a promotion list, they could put the (P), but you wouldn't sign correspondence that way. Only if you were in a position that you needed to, as the good 1SG (Join to see) has explained.
Its like asking if its OK to wear BDU's on your couch... Of course it is.