Interesting thought, Lee Evans may very well be a good receiver with his best days behind him. Which leads me to ask, if the Bills did want to move Evans would they get a decent return? For a receiver coming off of 37 receptions, 578 yards and 4 TD's I doubt the Bills could get higher than a mid-round pick at this point. If the Bills were to make the decision that Lee Evans does not fit into the Bills long term plans their best course of action would be to hope for Evans to have a comeback year and move him when his value rises.
To answer your question more directly, taking into consideration where Evans is in his career and where the Bills are as a franchise I would say that Evans is not "needed." That said, his financials are not outrageous and is a decent place holder while the Bills continue moving forward in their building process.