I have too much time on my hands and will rant a bit here. My thoughts on doing the 5310 are completely contrary. My viewpoint is that of someone who works mostly on micro plans.
I've been doing this long enough to remember when there was no fee for the 5310, and you got an FDL within a few months. Then they started charging for it, and taking longer and longer. It used to be something that we insisted on, then gradually became a client option, and now we barely mention it. The main thing that triggered my policy change was that we had a plan that was audited, after we received a FDL. I said "but but but but we have an FDL" and the agent said "yeah but we "just" need to review some operational things, and the FDL doesn't cover that." So they did the audit, and it was a major PITA because the company had shut down; fortunately they had some stuff in a warehouse. Of course they found nothing.
Unless you have an individually designed plan and need the FDL to cover the document, I do not think it is worth it. Basically you have a choice of 1) getting the FDL and spending a decent amount of money and waiting an interminably long time for an answer, and still being subject to an audit of the plan's operation, or 2) not getting the FDL and being subject to an audit of the plan's operation. You really should not have a document issue if you are using a pre-approved document.
I don't believe the ADPs and Paychex of the world are submitting 5310s, and they have way more plans.