Whether you're paying for independent publicity services, or your publisher is providing publicity support, there are quite a few points of logic that almost nobody will admit aloud.
1. You're Paying Me to Do a Lot of Grunt Work
While the most valuable thing your publicist has is her proprietary list of contacts, the job is messier and more clerical than you'd think. Plenty of bureaucratic admin goes into a publicity campaign: sending out the blitz / cover reveal / galleys / teasers, then following up with crucial contacts, then keeping track of who has responded and who has not. Unfortunately, unless your publicist has a crack team of minions, (and she probably does not) there's no separating the brain work from the grunt work.
2. If You Did This Job Yourself, You'd Learn a Lot
When you're the one contacting bloggers and reviewers, there's a lot of understanding to be gained. You'd read first-hand the bloggers' wants and needs. You'd find a few supporters out there who would prove to be invaluable as your career progresses. You'd understand just how difficult it is to break through to the voices who matter. It's uncomfortable to pimp your own book, but sometimes discomfort brings wisdom.
3. If You Did This Job Yourself, You Might Not Make Your Next Writing Deadline
The tradeoff is always time. No matter how easy it seems to be to hit "send" on an email list, good follow-up is time consuming. Authors who do their own publicity work understand this, and make time for it.
4. I Didn't Read Your Book
Publicists are overworked. They need to spend most of their time working the old email chain gang. Don't hold it against her if she hasn't read your book. In fact, don't even ask. Instead, be very clear each time you have a publicity suggestion that relates to the narrative. "Dear Becky, since my main character is a contestant on a Survivor-like show, let's make a big push on the reality show forums on Yahoo." Etc.
5. Once I Get a Satisfactory Number of Publicity Hits for You, My Attention is Elsewhere
Some books are just an easier sale than others. But the publicist has to look good to all her clientele (or in-house editors.) If she's lucky enough to score a bit hit for you (say... People Magazine. Let's dream big.) her next burst of effort will be spent elsewhere. We all need to rest on our laurels from time to time, because there just aren't enough hours in the day. An author who wants to be sure that she's still on top of the pile will stay in touch, making helpful (but not pushy) suggestions.
6. If You're a Sweetheart, I'll Try Extra Hard For You. (The Converse is Also True.)
It's just human nature. So check your ego at the door. Always be polite to your overworked publicist, and never lose your temper. Even when your publicist drops the ball on something. Even when your Amazon rank makes you want to cry. Punch a pillow, and then be kind to your publicist.