One of the strengths of the Fire Emblem series is its emphasis on units as valuable individuals, not cannon fodder. This uniqueness comes to the fore not only through support conversations, but also through gameplay: Matthew has personality not only as a character, but also as a frail thief; Gilliam has personality not only as a character, but also as a slow, stout tank. It takes me out of the experience to see Kellam as an assassin, or the proud Panne using a wyvern mount, or the shy Olivia chopping enemies up as a swordmaster.
Yes, these class branches are optional. I'd gladly sacrifice customization for cohesion.
Sacred Stones got this system exactly right. Its small cast is a strength: each unit gets a fair amount of screen time, each unit is well-developed, and each unit is mostly indispensable. Trainees aside, you're only getting one knight, one troubadour, one mage, one monk, one wyvern rider, one mercenary... if you're careless in battle, you're losing more than just a personality. Furthermore, the branching path system conforms to each character's identity. While it makes little sense to see Chrom as an archer, it makes perfect sense to see Gilliam as a great knight, or Neimi as a ranger, or Lute as a mage knight. Sacred Stones blends customization and cohesion in a way Awakening and the DS games did not.
Through its branching plot paths, FE14 provides the perfect opportunity to mix Sacred Stones' polished class system with the unit variety of other games. Assuming there is a Western, Eastern, and Neutral path, each selection could present the player with 20-25 units and, say, 10 units exclusive to each branch, bringing the total playable character count up to 40-45. Seems like the best of both worlds. Get rid of second seals and restrict infinite leveling to postgame.