The role of a yeast starter is to make sure they are viable, and get the yeast ready to do their job (gobble sugars!)
Would you rather pitch yeast into your wort knowing that it is alive and active ready to ferment your wort, or take chances on some washed yeast that you have no idea what the cell count, or viability of it is?
So while you may have a nice amount of yeast, regardless of how great your washing process is, you more than likely have a little trub and definitely some dead yeast cells.
You can save yourself some money by making a starter every time and use a smaller amount of yeast. Liquid yeast definitley costs more than 1lb of DME, and all it takes to make a 1L starter is ~6oz.
If you feel you have plenty of washed yeast then just allow that to warm up to pitching temps before you pitch it, but I am from the school of thought that a yeast starter, unless pitching onto a cake, is a must with washed/reused yeast.