In my experience, many law firms want to see your law school transcript at the outset. It is often a prerequisite to even being considered. For these firms, the very short answer is "No, you can't send just your resume if you want your resume to be looked at." More to the point, though, you almost always want to send your transcript when applying to a firm.
How much weight a law firm gives to the transcript totally depends on the firm. Some law firms have strict grading criteria even when they consider more senior attorneys. Some firms will not consider any transcripts that have anything lower than a B. Some only want to consider lawyers who graduated at or near the top of their class. Bear in mind that although many firms want to see (or require that they see) your transcript, it is a smaller percentage of firms that are as strict about grades as the firms I just mentioned.
Thus, the first misconception is that every firm that requires a transcript will not consider a candidate who has a transcript that is anything less than stellar. This is absolutely not the case. For most firms, your law school record is one factor among several that they will consider with respect to lateral hiring. What weight the transcript gets will depend largely on the firm, the partner who is looking at the resume, and the strength of the other facets of your background.
At the end of the day, if there is something about your transcript that might exclude you from consideration, the only thing you can do to mitigate this is to work with a recruiter who will explain (or you can explain) the context of the law school grades. Give the firm the opportunity to see the grades in the appropriate light.
It's not productive to be defensive or tell a firm that it's stupid to consider grades. It can be productive to provide a firm with relevant information about your law school career that might mitigate any less-than-stellar marks. You can and should highlight those qualities (your law firm experience, for example) that are so compelling that grades become a far less significant consideration, if they are considered at all.
I believe the biggest misconception is that if one removes the law school transcript from the application process, the firm will somehow assign it no weight or very little weight when considering his or her candidacy. I do not think that this is true.
If a firm likes everything it sees with respect to your background but has to ask for a transcript, the easy assumption to make (from the firm's perspective) is that you have something to hide. At this stage, you've highlighted the transcript issue for the firm, and I believe your transcript will get more scrutiny when the firm has to specially request that you provide it. Not only have you shined a spotlight on the very part of your background you wanted to obscure, but also the firm is going to have to find your initial application and make a complete file from what could have been complete at the outset.
What many candidates may not realize is that part of the application process is due diligence on the firm's part. Firms often have internal policies that dictate what they need to have to consider a lateral candidate at various stages throughout the process. You may fundamentally disagree that your law school transcript is relevant, but it may be little more than an administrative requirement that has to be checked off for you to move on to the next stage of consideration.
I hear a lot of frustration from lawyers about the transcript issue. I understand that a bad torts grade from your first semester of law school as a wet-behind-the-ears 23-year-old may seem to be a silly thing to consider for an associate position a decade later. I don't even remember my torts grade, and I would hate to think that my future hinges on it.
Some firms do find it important, but that may simply mean that you are not a match for those particular firms - and that's not the end of the world. However, I believe you'll find that if you understand why the transcript is used along with a variety of other factors, many of which will be far more significant to a firm, including it in your application materials won't seem to be as much of a negative as you think it is.