Helvetica is a great font. Arguably the greatest. But it is still important to remember that all things have their place. In the scheme of typography, Helvetica can be used well, but it can also be greatly overused.
Though most people don’t want to spend a lot of time thinking about which font to use for each writing occasion, a little understanding of typography can be very valuable in making sure that the fonts you choose help you convey the message you want in each medium in which you write.
Attorneys should know by now that Helvetica (or Microsoft’s bastard spinoff, Arial) is a terrible, terrible font choice for use in legal documents. Book fonts (i.e., fonts with serifs, and not newspaper fonts like Times New Roman) are far more appropriate. But don’t take my word for it. The 7th Circuit Court of Appeals has a excellent typograpy and legal writing guide for lawyers. Please take some time to understand it.
What should you use? Many people think serif fonts (like Times New Roman) are easier to read than sans-serif fonts (like Arial or Helvetica) on paper. The general wisdom is reversed when it comes to computer screens. But because some of your audience (federal clerks) may be reading your briefs onscreen, and some (judges) will probably be reading on paper, I think it makes sense to try fonts like Georgia and Constantia, both free Microsoft fonts, made to look good on paper and screens.
You may also find value in the book “Typography for Lawyers”, which discusses Helvetica and other font selections, in addition to other legal writing and formatting advice.
However, Helvetica is still a very popular font for graphic designers, and for good reason. There is even a fairly entertaining movie about Helvetica. But it quickly loses its appeal in long passages of text.