While clarity is certainly a virtue in philosophical writing, all else being equal, I think there's a distinction to be made between writing clearly and writing well. One can write clearly and still fail to write well. Many philosophers manage to be perfectly clear but are stylistically drab, dull and ponderous. Other philosophers may fail to be particularly clear but instead write enthrallingly.
The best writers can be clear and engaging at the same time. There's a beauty and an elegance in really good writing that neither adopts the blunt, clunking, mechanical tone of a phone-book nor gives in to self-indulgent, quasi-poetic flights of fancy. Neither extreme makes for good communication, which is what really counts.
As a rule of thumb I feel that a piece of writing is well written to the extent that it is rewarding to read -- consequently a lot of it depends on the quality of the ideas. I've read many things that were perfectly clear but bored me to tears and, worse, lacked any redeemingly insightful content. Equally, I've read things that I didn't immediately understand at all but which gripped me enough to make me do the hard work to come to understand them.
The very worst kind of writing is the sort that wantonly wastes its readers' time, mindlessly pummelling them with ugly, unnecessary, vacuous verbiage. I find writing of this kind to be disrespectful and arrogant -- writing to be read is a privilege, not something to which anyone is entitled. No one should have to wade through stodgy, artless, unedited word-stew just because the author can't write properly.
If your book takes 10 hours to read and 1000 people read it that's 10,000 hours of human existence -- of living, breathing human life -- that your writing has consumed. Yes, usually people choose to read what they read but this is not always the case. Regardless, I think it's the responsibility of the writer to make those 10,000 hours as rewarding and useful as possible -- and clarity is but one factor in that equation.