How much is lost by not strictly following the prescribed armies that nations/factions historically fielded?

The last I can remember attempting to stay true to one of those rubrics was in a .81 Romani game, primarily since the forum documentation for it is so complete. I got sidetracked before I could even get to the Polybian reforms, but I do remember having a blast being outnumbered in virtually every battle of import and relying on the tenacity of veteran cohorts to maintain a steady line rather than fielding enough units of men-with-pointed-sticks to tie up an enemy so the heavy elite infantry can huff and puff around the flank and end every battle in under five minutes.

In my current Baktria game though, I haven't followed any sort of rubric. Don't get wrong, I'm not fielding full stacks of pezheitairoi led by five generals, but I'm not taking the same anal-retentive attention to detail a Warhammer fanatic takes to the ratio of his companies of Skaven head-hurlers to Orkish oliphaunt riders or somesuch.

What is the EB team's take on historically composed armies- are they an essential for a complete game experience? I imagine piecing them together would be a lot easier for some factions rather than others-- the game is building around the countless lacunae in both the archaeological and written records after all.