The notes on either side of a special note are called "shoulder notes". Examples would be BFD1112300 and BFD1112302 on either side of BAX8777881, or perhaps BBK1234566 and BBK1234568 on either side of the ladder note.
In my opinion, the BFD notes in the example above would not really be worth keeping as they have no relation to the BAX note. Looking at the BAX note, there's nothing to say that it was really those BFD notes versus two other notes from a different prefix.
The run of three BBK notes however would be good to keep together. Similarly if you had an error note with its two shoulder notes. Some collectors find value in having these neighbouring notes, while others do not. To each their own!
Personally, I have a couple situations where I've kept the shoulder notes. The two that come to mind are actually Canadian Tire Money. The first is a set of four notes with the middle two being error notes. The second is a set of three again with the middle note being an error. In both cases, all notes in the run are sequentially numbered - none are replacements. I think I have an example in my Canadian collection too but I can't recall what it is.
Generally, runs of sequential notes tend to depress the overall value of the notes. One possible explanation is that potential buyers see that there is a plentiful supply versus demand thus driving down the price -- basic economics.
Of course all this is just my opinion... Others may have contrary opinions and that's perfectly fine.
Logged
BWJM, F.O.N.A.
Life Member of CPMS, RCNA, ONA, ANA, IBNS, WCS.
President, IBNS Ontario Chapter.
Treasurer, Waterloo Coin Society.
Show Chair, Cambridge Coin Show.
Fellow of the Ontario Numismatic Association.