we have a dressing area and en suite - I love the dressing area because it means the bathroom isn't directly off the bedroom so if you go to the loo in the middle of the night it doesn't disturb the other person as much.
We don't actually have any cupboards in the dressing room - it's a funny shape and is effectively only full height along one side, the rest the roof slopes and reduces the head height down to a couple of feet, so it's more of a glorified corridor with space for a couple of low cupboards/drawers.
We also have, which I love, the wall between the bathroom and the bedroom is a built in cupboard, and the door into the dressing room is a normal door - so you're not going through cupboards as it looks in the picture somebody posted previously, the cupboard is behind a proper wall with doors rather than just a run of cupboards. However it's great - it acts a another layer of sound insulation between the bathroom and bedroom.
if you didn't want to have the door to the cupboard in your bedroom, you could just have plain wall and then leave a gap that's big enough for a walk in cupboard rather than a full on dressing room to provide separation between the two, have access from between the bedroom and bathroom, so that you can still shut the bedroom door and access both the cupboard and en suite separately. You don't want to be in the position of not being able to get into your cupboards because someone is on the loo or having a shower! Also if you don't have great ventilation then it's not going to be great for storing clothes in case they get damp and mouldy.
The other thing to remember is that you don't always need to have straight walls. In my guest room, the small en suite has a shower at one end, the loo at the other, the sink in the middle. The width at the loo end is wider than the shower end - which is just the width of the shower cubicle. The door is on an angle and then the loo bit has straight walls again - so it doesn't take up too much unnecessary space. If it took a rectangle out that was the width of the loo end, it would be really intrusive - this way, really not too bad. So playing around and putting the shower or bath or a loo area that sticks into a dressing area or the bedroom can help you to play with the space much more effectively than thinking you have to have straight walls.