Expanding on the point made in Brad Feld’s latest Term Sheet post re over-lawyering – a term sheet, useful tool as it is, is really nothing more than a handy way to keep track of the key deal terms the business people have agreed upon (in principle). It’s not a binding agreement; it’s a way to determine if you can even get to a binding agreement, and once there, a tool to facilitate the negotiation process.
As such, there’s no need to draft it with legal precision - that can wait for the definitive agreements. Sure, there are cases where you need legal input on key deal terms that are inherently "legal." Some deals will have key terms involving specific IP rights, or apportionment of environmental liabilities, or some other nasty thing that you'll want your attorney to review/draft. Other than that, if you're going to insist on having attorneys review or draft the whole thing, you might as well dispense with it and go straight to the definitives. I have been involved with plenty of deals where no term sheets were exchanged, and many where the “term sheet”, such as it was, consisted of nothing more than an exchange of e-mail. While it's true (as Brad points out) that more detailed term sheets are generally better, at some point the additional detail is outweighed by the cost in time spent at the term sheet phase.