Are party members expected to be disposable and swapped out when I feel like it or they die?

I'm accustomed to playing later Bioware games like Knights of the Old Republic and Dragon Age where you can recruit every party member and swap them out as needed. I'm really confused about Baldur's Gate (Enhanced Edition, if it matters), where I am getting hordes of potential companions and I'm being asked to permanently kick some out.

At the beginning of the game, Imoen followed me and asked to join. Then I met Xzar and Montaron on the road and they joined. I made it to the Friendly Arm Inn and met Jaheira and Khalid, giving me a full party. I went to Beregost and met Garrick, Kagain, and Neera, all of whom wanted to join my party, but it meant that I had to kick out people (permanently, as far as I can tell), or let the new person leave.

Every time I had this happen, I decided to load my game and just avoid the encounter so that I wouldn't be forced to lose someone (I was becoming rather attached to my companions). I also chose to reload the game every time someone died, which they sadly do a lot, so that I wouldn't lose anyone. But after this happened with three different potential companions, I'm wondering if my expectations are wrong.

Am I not supposed to be so attached to my companions? Are they expected to be disposable and just replaced when I feel like it or they die?

As originally designed, it was expected that players would lose party members on a regular basis and need to replace them. This is why there are so many NPC options, why they become available at various points throughout the story, and why so many of them are thieves (the game is much harder if you don't have a thief in your party). Turns out, most people don't play like that: they form a party early on, and do their best to keep it intact throughout the game (this also generally gives you a stronger party). This is why Baldur's Gate II has fewer NPC options, and heavily front-loads your access to them.

If you turn someone down when they offer to join your party, about half the time you can return later and ask again. The exceptions from the original are Dynaheir (and if you refuse her, Minsc leaves), Yeslick, Branwen, Jaheira and Khalid, Montaron and Xzar, Viconia, and Shar-Teel. I don't know about the NPCs added in the Enhanced Edition.

If you remove someone from your party, they will usually stay where you left them. You can then talk to them later and invite them to re-join your party. I've seen reports that if you leave them alone long enough, they'll be "reset" in equipment, location, or both. It's not clear if this is a bug or not.