Julie –
Rarity of arbor-spaces: a very good point but they are not too difficult to make to sufficient accuracy for all but the most exacting work.
Apart from conventional turning from stock, an alternative approach is to use gauge-plate or similar. For some applications they could be rotary-sawn from stock mild-steel plate or sheet, then tidied on the lathe. The bore and o.d. are not too critical.
The advantage, and point, of a set is that one can accurately set any distance, at least to a few tenths, but choosing appropriate spacers. The sizes of the spacers are similar to gauge blocks, allowing one to build up any length needed in small increments, half a thou for my set, I think. Side milling horizontal cutters are not perfectly ground, so being able to adjust the distance in small increments allows one to adjust for cutters where the cutting teeth are not perfectly aligned with the centre boss.
The reality is that in decades of machining I’ve never needed to gang mill on the horizontal. But I like owning quality tooling, I am learning a lot about the set and their use, and if you haven’t got a particular tool you cannot plan to use it.
Julie