Classification is a system of coding and organizing objects, be they books, species or whatever. In the library stacks, we expect to find books with largely corresponding subject matter shelved together. Similarly, in a local, regional or continental flora, we expect to find similar plants 'shelved' together. Placement of Penstemon and allied genera in the Plantaginaceae is the funcitonal equivalent of placing floras in with the cookbooks: it does not serve direct discovery.
Users of floras use floras because they want to know the name of a plant. Although polyphylletic, the "Scrophs" served this objective. Find a tubular corolla, often brightly colored, often 2-lipped, capsular fruit plant and fish about in the key to Scrophs...
Taxonomy and classification are often incorrectly equated, to do so is to conflate. Taxonomy is a system of classification, A taxonomic system is a particular type classification: a classification based on phyllogenetic relatedness is one system. An alternative 'system' often used in flora guidebooks is flower color.
Placement of Penstemon, Collinsia, Kecklilla, Tonella (California plants) and others (Chelone, Chinophila, Russelia,
Tetranema etc) in the Plantaginaceae, along with dozens upon dozens of equally diverse tribes and genera creates a discordant grouping. An expanded Plantaginaceae is a discordant grouping. Recognizing Plantaginaceae becomes difficult. What key feature does one use to equate these plants? In my opinion, none!
The point: in a regional flora, if one is such dire need to use a phyllogenetic system, why be bashful? After all, we no longer grumble about splitting up artificial, aggregate genera (vis Stipa into many genera) although sometimes revert to these aggregates.
Penstemon is a member of the Chelonaceae Martinov, containing Chelone, Chinophila, Collinsia, Keckiella, Pensteomon, Russelia, Tetranema, Tonella and Uroskinnera. All are New World, bird or bee pollinated, large showy flowered (or for Collinsia, unusual in their "pseudo-papillanceous flowers). Placement of Brookea auriculata of Asia is the only outlier in that genus should it belong here.
The problem with an expanded Plantaginaceae is that is a library where no two books shelved next to one another share much information. Make families that are closely related lineages consisting of groupings of genera and their radiant species. The expanded Plantaginaceae is not a natural family.
California botanists, recognize the Chelonaceae.