it's not about getting soemone else's taste, well, not unless they are a very bad interior designer.
the idea is that you use them as an interpreter. you say 'i want the room to feel warm/stylish, i like art deco/bauhaus/arts and crafts, i listen to lily allen/lambchop/the damned and i want the room to fulfil x and y and z purposes... then the interior designer will come up with a few interpretations of those concepts.
we just knocked down two walls in our flat and put holes in the rest, moved the kitchen and bathroom (new designs in both) and put a mezzanine and a loft space in as well. we had an architect who drew up my plans and i did everything else, layout-wise.
it all seemed terribly obvious to me, i would show people round the flat as it was and say 'so there won't be a wall here, here's where the bedroom will be (pointing to the kitchen and bathroom) and we'll put stairs up there to the mezzanine' etc etc. and as i spoke, i could actually see it happening in front of my eyes.
however, i reckon that of the people i told, about one in fifteen had a clue what i was talking about and one in ten could properly visualise it. the rest simply could not see it, because their brains work differently to mine. my dh, for example, could not work it out at all and just needed to trust me while i demolished his home.
given my experience, i think a lot of people could benefit from an interior designer, while there are some who just don't need the input as they can see it themselves.
but if you've got bare magnolia walls after four years, HyacinthB, you sound to me like someone who might enjoy using a designer. (and remember to get them to tell you about their discounts and see if they can pass some of them on... and definitely buy some interiors mags.)