A little historical perspective might be useful. The Inexorable Search is a spell that's been around since uhmm ... certainly third, possibly second edition.
At that time the ArM5 system of "take base guideline, add modifiers for R/D/T" was not in place. You just thought what you wanted a spell to do, checked that it did not violate the Limits of Magic, assigned it Technique and Form, and eyeballed a level that seemed "right" in comparison with existing spells. So, nobody thought about shaving D:Conc down to D:Mom (which would have been D:Inst, if I recall correctly) in The Inexorable Search, because the mechanics did not tell you that the D:Mom version would be one magnitude lower: in fact, since common sense told you that they would be roughly of equivalent usefulness (for the reasons you cited) and D:Conc felt "right", D:Conc it was.
When ArM5 was published, a lot of "old" spells where converted to the new system. In most cases (with a few notable exceptions) the process involved taking the spell as it was (with the "original" R/D/T, and the "original" level), and setting a generic base guideline at the level that would yield exactly the "original" level when adopting the "original" R/D/T. Of course, in a number of cases, this resulted in spells that were not as efficient as they could be. You can address this in (at least) two ways: a) either disallow the "magnitude savings" (remember that you can always adjust the level of a spell if it does not seem right, so you are perfectly within your rights to say that a D:Mom Inexorable Search has the same level as a D:Conc one) or b) point out that magic theory has evolved over the centuries, so there are some spells that the Order now could re-invent in a more "efficient" fashion, but they are such "classics" that all Lab Texts, Mastery books etc. have them in the old format.
The case of the Inexorable Search shows one more quirk about this process of "adaptation": sometimes the guideline for the "old" spell can be used in a totally different fashion with different R/D/T. The Inexorable Search, cast (most likely spontaneously!) at R:Self, becomes the Bane of the Maze, and allows a magus with a map to unerringly know his location on the map.
I would end up by noting that, although there are some drawbacks involved, if you really, really want to cut levels on "The Inexorable Search" (and similar spells), a very effective way to do it is to Open the Intangible Tunnel to the target (R:Arc, D:Mom, T:Ind, ReVi Level 3) and follow with a R:Touch, D:Mom version of the Inexorable Search that ends up being only Level 4. Spontaneous magic, possibly even non-fatiguing, is all you need unless you have to pierce Magic Resistance.