In fact who needs more than 22? ...What do people see different on ultra high resolution backs other than resolution? How high a factor do you rate analysis for the quality of your photography...?
Reasonable question if you do not expand the applications of these backs into areas you may not be familiar with ... a few touched upon by Bob.
Many folks (like myself), were introduced to MFD through 16 meg square backs like the Kodak ProBack, or 22 meg rectangular backs ... both of which featured a 9 micron pixel pitch affectionately called "Fat Pixels". These backs produced a very pleasing aesthetic that some still favor to this day.
However, certain short-comings like moiré, saw-tooth edges when enlarged for outlining, plus more and more demanding client multi-use applications called for more resolution or fidelity. For example, many print ads are often reproduced in a huge array of final trim/bleed sizes and page ratios, from digest to tabloid. So for example, a still life spread shot of food would have to have a huge amount of non-critical back-ground included ... previous to higher res backs, this was often solved through expensive retouching.
To help this, there were innovations like micro-step (Multi-Shot) 16 or 22 meg backs ... featuring 4 and 16 step shooting, (which I now see you are familiar with).
IMO, what the 33 and 39 meg backs offered was a more mobile one shot solution. The Multi-Shot versions continued being produced in these higher res backs for those needing it ... including demanding commercial, industrial, institutional and museum applications. I had a Hasselblad CF39 Multi-Shot I used to shoot GM work with, but could still shoot single shot for more normal stuff. The difference in tonal subtleties, color fidelity, and detail between the 39MS and 39 in single shot mode where dramatic when viewed at any distance ... not subtile to any eye.
When the H4D/60 was made available I asked for a test to determine if a 60 meg, slightly larger sensor was enough to do my commercial work, but avoid the tethered, "locked down tripod" discipline with only static subjects required of Multi-Shot. It was close enough in all areas ... except M/S color was still better. The bonus was that the higher res back combined with the well known Dalsa skin tone aesthetic helped take my single-shot portrait, and environmental portraiture to a new level with wonderful color separation and subtile tonal transitions.
DR is very good with most of these backs, but for many commercial shoots the dynamic range in a scene isn't left to chance ... it is all lit and balanced.
For most, MFD 33 to 40 meg is the sweet-spot for what and how they shoot. I like my S2 for most work these days because I'm backing off commercial work and all the possible demands it entails. However, I do not confuse the H4D/60 output with that of the S2.
-Marc
added after reading more responses: If I were still in the thick of it rather than backing off in semi-retirement ... I trade the H4D/60 for a H5D/200 ... giving me a 50 meg back for single shot, 4 shot for some work, and 6 shot for the most demanding work.