As annoying as they can be, I think Sheva and Ashley are interesting and necessary components of RE 4-5 because they re-introduce the importance of time & space control into games that desperately need it.
Two consistent issues that pop up in modern action games is how little meaning they give space control and time. Arenas tend to be open, your movement tends to be quite freeform & enemies struggle to catch up if you decide to run away. Furthermore, running away doesn't mean anything - its either neutral because you won't lose much, or positive because it resets the situation when you've lost advantage.
Here's where everyone's favorite dopey companions come into play. They chain you to a constantly moving "pivot point", which is vulnerable to attacks. You're given an annoying, intrusive "tail" which can put you at risk if you decide to carelessly retreat, because unlike Leon/Chris, they won't be able to dodge enemies as effectively as they're running away. Additionally, focusing on yourself can leave your companions cornered, it gives evasive/defensive play a cost. The situation between you and the enemy might remain neutral, but the situation between one of the ladies and the enemies might be quickly going to shit. The game essentially recognizes its enemies, spaces & mechanics can't really pressure the player very well, so it has them pressure someone far dumber & slower.
Of course this is all still very, very soft. Realistically you can run away if you're a bit more careful or do some light planning, and the pressure is fairly minor most of the time. The much better way of handling this lies in Mercs mode in the form of the combo & timer. But if devs don't have the stones to bring back timers, strict space limitations or building games around scoring, the least they could do is bring those elements back in softer ways, instead of making companions an invincible non-element.
