So far I leveled 3 characters to 50: Operative healer, Marauder and Juggernaut (all female) and have a bunch of characters around lvl 30.
In all cases, I find myself picking up one companion that I take with me everywhere, while neglecting the 4 others almost completely : that was Vector for my operative and Quinn for both my sith warriors, and I will probably do the same with future characters.
I also raised affection with Vette, Jaesa, Kaliyo and Temple with gifts to discover their stories, but it doesn't feel the same as actually fighting alongside your companion. Plus, in your class story you get some additional conversations with your current companion, so you get to know them better ofc.
I'm happy that my character's love interests also proved to be the most efficient companions when it came to leveling and taking on elite mobs. Being able to use your favorite companion all the time really adds to the fun