Too many of us own many DE razors because of the trial-and-error process in our first couple of years DE shaving.
Yet there are reasons to acquire and keep some razors with different shaving characteristics. These include shaving longer hair, using different razors for different passes in the same shave, and matching razors to the characteristics of a given blade at a given point in its life cycle.
To some degree, these multiple-razor arguments also fit for owning adjustable razors such as the Progress, Future, and Vision models from Merkur, or the vintage Fat Boy and Slim adjustables from Gillette.
For shaving long hair such as removing or trimming a beard or mustache, or cleaning up the back of one's neck between haircuts, or removing long neglected body hair, an open-comb baseplate can do the trick. Regular readers of my blog know that my favored open-comb razor is the Merkur 15C. I like this one because it's a mild shaver, not too likely to bite, but completely adequate as both a long-hair shaver and a regular shaver if one likes a mild razor.
Other razors that are good for shaving longer hair are the slant razors such as the Merkur 37C. These types of razors have large capacity -- in part owing to their toothed safety-guard design, which are essentially very deep, square-topped scallops -- but may be more harsh on sensitive skin than a mild open-comb razor. Also potentially adequate for shaving longer hair are the razors with a large blade-bar span. These, however, are potentially even more likely to bite than the slant designs. Of course, any adjustable razor on its more aggressive settings are likely to be equivalent to non-adjustables with large blade-bar spans. The only difference is that most adjustables on an aggressive setting will also have a rather positive blade exposure, which are more likely to wound; while non-adjustable razors with a large blade-bar span may or may not have a positive blade exposure. This means that a non-adjustable with a large blade-bar span but a less-positive blade exposure may be desirable for those who want large capacity (such as those who shave infrequently) but have skin that is easily wounded.
Another argument for owning razors of different shave character (or adjustables) is matching the shaving character of a razor to a given brand of razor blade. Notoriously sharp and unforgiving blades such as Feather brand might work best in a very mild razor, in which the blade is less likely to nip.
Perhaps even more compelling is matching the razor's shaving character to the point of a given blade's life cycle. For example, my preferred blade, the Personna Platinum Chrome, when fresh out of the wrapper may give me the best shave in my mildest three-piece razor, my Merkur 33C Classic. Yet on its fourth shave, it may perform best in my Lord L.6 razor head, which has the same blade angle as the 33C and a larger blade-bar span, but still has a negative blade exposure. The eighth shave on this blade may be best in my Merkur 15C open-comb razor, and the fifteenth shave may be optimal for my face in a Gillette Tech or Rimei RM2003.
Similarly, using a Gillette Slim adjustable, the first shaves might be set to one or two, the fourth shave to three or four, the eighth shave to five, and so on.
The final argument for multiple razors is using different instruments (or different adjustable settings) for different phases of a single shave. When using non-adjustables, I may still select a given razor on a given day according to blade-life-cycle considerations. However, for the final pass of a shave, I frequently choose my mildest-character razor, the Weishi 9306-F as a finishing razor that is the least likely to bite (but still will, if one gets cavalier and careless). Some even use three razors for a given shave, using a progressively milder razor for each successive pass.
The adjustable razors fall slightly short in this matching-razor-to-phase-of-shave approach, only in this respect: adjustable razors may not adjust mild enough to match the character of the 9306-F for use as a finishing razor.