User Agents have proven to be (somewhat) uniquely identifiable under specific conditions so it seems like a reasonable precaution regardless of Silk Road configurations.
He can specify his own User-Agent string. If he sends the user agent 'silkroadparserbot v0.0.1' with every request, that is going to provide no information at all.
Even if this wasn't, did you think that after one change of UA he would be getting more 'uniquely identifiable' with each new request sent?
Even if this wasn't, did you think that after one change of UA he would be getting more 'uniquely identifiable' with each new request sent?