Well, there are few reasons why I myself will never go for small scale. Two major reasons are difficulty level and campaign progression.
With the frist one, game is unplayable on legendary as it forces you to play with VH battle difficulty, which means +7 to most of AI units stat. That makes levies rout your hypaspists. That is not fun ; P That plus most of people not realizing that higher battle difficulty = more AI stat bonuses and they spam threads about how unit x routed y this mod sucks etc

Additionaly with higher scale you can play with Hard battles and have fun as AI units will present a bit bigger challange but not unbearable. The same goes for VH with larger scale, it is tough but manageable if it is someones cup of tea.
As for campaign progression, units get various stat bonuses from exp, army traits, general traits, tech, building and so on. I know that it might be realistic for some super hardened hastati with all shevrons possible to go toe to toe to some elite units or have comparable stats. But that does not work for gameplay as player has no need for upgrade. You basically go with some starting units up till the end of campaign because they can go to to toe with elite units that all musters. That with having some provines fully dedicated for OP units produced armies that were simply to strong for AI to counter as AI is unable to both present any tactics in battle and nurse its units to that kind of training and equpement. That aspect is even more improved for 1.1.