Sure. After a busy Class D Tower or Class C controller tells you to fly a heading, maintain an altitude or enter the pattern in a certain way, do something completely different, tell the controller you intentionally didn't follow the instruction because it was only a recommendation and you didn't have to. Let us all know how that works out from a 91.123 enforcement standpoint.
I've done my share of VFR flying in C, D and E airspaces, and
other than entering or leaving the pattern (!!!), have never gotten any directive to fly a specific heading and altitude, for any reason. In fact, I have rarely had any suggestion offered for traffic avoidance, other than a bearing to the traffic and its altitude. I've been asked to follow this or that, hold here or there, report here or there, remain clear of this or that... been
asked if I would please climb or descend or veer this way or that... but never have I been told "XXX degrees" and "XXXX feet."
If I did, I'd probably comply, but I might at least dare to ask why. ATC does not reign atop Olympus... their power and authority have limits. They also make mistakes. I've been placed in jeopardy more than once by blindly trusting controllers to have a clearer view of the Big Picture than I did. It is not always so, and the regs acknowledge that you have the right to go "huh?" if compliance could create a hazard.
If I ever get called on the carpet for, say, questioning or refusing a controller's "instructing" me, when
enroute ,under VFR, in a Class E, to fly a specific heading and altitude that would take me right into a Bravo without a clearance, I will have this quote handy (from the letter cited in your own post):
"If the pilot only received the vector for traffic from ATC, the pilot did not receive a clearance or instruction from ATC. Therefore, any maneuvering by the pilot is not a violation of § 91.123."
Get it? "Fly heading 030 and maintain 2000" for a VFR flight
enroute in Class E (or even C or D) airspace is
not a clearance or instruction, whether intended for traffic avoidance or any other purpose. Technically, it is a request. Arguing that "you're better off just doing what they say" is invalid, in my book. I'd rather get yelled at than actually bust a reg... or end up dead.
I don't expect them to say "pretty please", but I would be dubious... in fact, my first thought would be that it was an error; that the controller had me mixed up with some IFR flight or forgot I was VFR.
Pilot and controller must understand their roles thoroughly, or safety is compromised.