One of the arguments often served against having a water proof breathable detachable inner liner is that the outer shell becomes drenched and heavy. This school often reccomends a breathable waterproof shell (which will aborb some water, but not as much). Problem with the latter solution is that once the shell is drenched, it also stops wicking sweat - possibly drenching you from the inside. Also, you loose the option of removing the water proof shell on hot sunny days - drenching you from the inside on hotter days as well as the jacket is much hotter. In colder temperate climates, this may not be such a big problem as you might not sweat so much, if any at all, and the "drenched shell" argument may come into play.
Having recently been testing both types of kit (Bullfighter VS BMW), in various conditions (hot, cold, rain), I am now in the market for a kit with a detachable inner... I've set my eyes on the Rally 3 from BMW. It is in my opinion the best compromise.
Keep in mind that many jackets with non water proof shells, including the bimmers, have had the nylon fibers treated with waterproofening from the factory, in order to not soak up as much water. So, it is not like it gets drenched immediately... Remember also that even with the water proofening built into a breathable outer shell, the shell will hold some water. Only an impenetrable outer shell will prevent cooling in my experience. So, if it rains in a cold area, the one-piece has this added benefit - it keeps you warm as well as dry.
In areas with probability of some rain, I would keep the liner zipped in at all times and bring a one-piece outer to pull on if it was to pour buckets. In hot dry climates, with low probabilty of rain, I would leave the one-piece at home and put the waterproof liner in my pannier.
For instance, if you are to ride across the atlas mountains, you will likely not have rain at the beginning or end of the day - but very much so in the middle, where it is also quite cold. Knowing this, I would keep the liner zipped in for this day. On the other side of the mountains, I would put it away as I would likely not no longer need it... but I will really enjoy a less warm jacket. In Norway, I would keep it zipped in on most days, except if the forecast underscores that it will be cooking. And, for any longer rides in Norway, where it can rain upwards, downwards, and from all directions at the same time, the one-piece would be near by if the forecast says that heavy rain is to be expected.
As for rain gear, I have experienced that with a two piece, water can creep up between the jacket and pant.. I have nov have a one piece which is dirt cheap, with neon panels for added visibility in crap weather... it has never let med down, in any kind of wet storm (snow, hale, rain). I think the most important attribute are long weather protected zippers way up the side of the legs and in the front so that it is easy to get in and out of without having to take your boots off. I really liked the tip of using plastic bags over your boots to make them slip through the legs easier - I will really test that one out for sure.
As for hands and feet, I have not found a solution that I like yet. If it rains I don't want to get out of my boots on the side of the road to put on (or take off) goretex socks, sealskinz or plastic bags. I have tried som rain mittens to put over my glloves, but feel they provide me with poor grip. I have tried some industrial rubber gloves which work well over thinner riding gloves, but not for bigger bulkier gloves. The same with the rain socks that you put outside the boots, which for me allways tear on the foot pegs anyways. This much said, I have only tried a couple of cheap solutions - so any advice on solutions would be greatly appreciated.
So far I have been riding in leather or synthetic hiking boots with goretex - I find them all clammy in hot climates, but ok still, and most have held up in rain. But, with more and more offroad riding, I am looking for a sturdy Enduro type boot - and from what I hear, few of these are water proof.
Any suggestions for keeping hands and feet dry???