For me he doesn't fit my evil playstyle as I prefer playing the scheming deceptive evil rather than cackling megalomaniac. I felt like my PC would constantly have to try keeping him from playing our hand: "Astarion, shush! We are not going to pick a fight with this community out in the open where we are outnumbered 50 to 1. Let's play nice and earn their trust and then backstab them when we have the odds on our side." And then Astarion just rolls his eyes and moans about how dreadfully boring we are. Like fine, I'll keep you in camp when we talk to people then and I'll let you know when the spilling of blood is about to begin.
For my canon playthrough where I had no metaknowledge beyond parts of act 1 that I had seen in early access and I played my character to react to things making the most logical choices that fit their personality and what they knew, I sent Astarion away after he tried to bite me. My logical reasoning for this was that I already knew him to be dishonest, so there was too much risk trusting that this time he would stay true to his word.
Trusting him turns out to be a very beneficial decision in the end, but it is objectively a terrible decision at that pont in the game when looking at all the facts you have in front of you.
