I have a different opinion.
There are no rules for or against it. No one will judge you or your script as being amateur because of "BEGIN CREDITS." All that "don't direct the film" or "that's the actor's job" stuff is film school/screenwriting book propanganda. As the writer, do whatever you must to communicate your story to the reader and elicit an emotional response.
I totally agree with creativeexec. As I mentioned on another thread recently the concept of "don't direct from the page" is so misunderstood, that many writer's don't tell the story from the page for fear of treading on the director's or actor's territory.
What pegs a writer as an amateur is the writing. Poor dramatization, lack of character development, dialogue that is bland, on the nose and lacking in subtext, no thematic clarity, lack of structure, lack of conflict, . You can normally peg an amateur script in the first few pages.
I also agree with CE as to the "why" of including "begin credits". If there's no good reason to do it, then don't do it, but if you have a reason then by all means put it in.
A true "spec" script is a "speculative" script or a "speculator's" script where the writer is hoping/praying/speculating that somebody somewhere might like his script enough to actually produce it -- which means he is a super duper low-on-the-totem-pole writer. He might be a good writer, he might be a previously-sold writer, but he's still just hoping/praying/speculating. So as long as he's standing firmly in the world of "spec writing" he has to play by the rules that govern such scripts.
I'd be quite curious as to where you got this impression, Plot. While writers as a group, tend to be lower on the totem pole than directors, actors and in some cases the craft services guy - I have never known spec writers to be lower on the pole than writers for hire, in fact they might get treated a little better - they're selling a completed commodity as opposed to trying to get work. Optioning a spec costs less than hiring someone to write a script (at least it should).
A first time out spec writer can sell a script for seven figures - rare but it does happen, however a new writer will never get a million dollar fee to write a script. They have no track-record.
I worked in development for years and couldn't even hazard a guess at the number of specs I read and I was never looking to see if the writer had the audacity to direct from the page or break the rules. I was looking for a good story, one that hooked me on page one and carried me through to page 110. I was looking for commercial viability, castability and whether the project fit into our slate. And whether I was developing a spec script or a project that originated in-house - there was no difference in the way the writers were perceived or treated.
I've also had numerous scripts optioned by different producers and I've been hired by several other production cos. to write or rewrite projects - and there was no discernable difference in the way I was treated between the optioners and the hirers - with the exception that the optioned projects had considerably larger paydays should the purchase go through, than the assignment jobs.
There were rewrites performed on the optioned projects. If anything the producers on the optioned scripts were less adamant with their rewrite ideas on the optioned scripts than on the projects I was hired to work on. Perhaps it's because the original story was mine, or maybe it was because the project was optioned, so at that point, the producers didn't own the project, and statistically speaking, there was a good chance the rights would eventually revert back to me.
Your job as a spec writer is to a craft an entertaining, dramatic and commercially viable screenplay. Your job as a writer for hire is to craft an entertaining, dramatic and commercially viable screenplay. The only real difference is that when you're hired to write a project you may be provided with specific mandates regarding what should or should not be in the script.