Removable liners can be really nice if they are designed well. That's a big IF in the world of ski gloves.
The benefits of removable liners:
1. The liners can be used by themselves as an effective second pair of light mittens/gloves.
2. The shells can be used by themselves as an effective second pair of gloves when it gets warm.
3. Once removed, removable liner gloves dry faster.
4. Often, removable liner gloves breathe better and are warmer because the liner and shell have more space between them (less condensed).
5. You can replace the much-less-expensive liner without having to buy new gloves.
6. You can fix the glove much easier.
The potential problems:
1. The problem with removable liners is that they are often difficult to re-seat in the correct position inside the shell. And when your hands are sweaty, it could be big trouble. That's less of a problem with mittens, but gloves can be a crap shoot, depending on the design of the glove. When you try them on at the store, make sure they come apart and go back together really easily, because when you're out in the elements it will be much harder.
2. The liner is not fixed so the gloves rarely fit exactly right, and dexterity is often decreased. The liner (in most designs) moves relative to the shell every time you move your hands.
As a glove designer who has spent a lot of time trying on and testing other company's designs I am constantly amazed by the terrible functionality of most removable liner gloves. Several prominent brands (which I will not list here) are barely usable at all. They are so hard to put on and take off AND re-seat once removed, that I wonder if any person from that company even tested them at all before pawning them off on the consumer. Also, I don't understand why a ski shop would carry a ski glove that you can't even get on your hand. Lots of removable liner glove designs should have been rejected outright.
Knowing all of this, Free the Powder designed removable liner gloves and mittens that fix all these problems. They are called the RX and SX models. They go on and go off as if they were sewn-in with all the benefits of a removable liner.
Sewn-in liner glove benefits:
1. Simple design.
2. The liners remain mostly straight and seated properly inside the shell for their entire usable life.
3. Never lose the liner.
4. Easier to take on and off when your hands are sweaty.
Sewn-in liner problems:
1. Slower drying.
2. Often not as warm. Less air flow between liner and shell.
3. If the liner has a twist, it's never gonna change.
4. You cannot replace the liner. Once the glove is packed out, you'll need to buy a new pair of gloves.
5. Really hard to fix. Will need a professional or a new pair of gloves.
Ultimately the choice is very personal, depending on how you use your ski gloves. For this reason, Free the Powder makes a variety of gloves with different designs, some with removable liner and some that have fixed liners.