As long as your expectations are realistic I dont see why entering to get back into the "competitive environment" would be a bad thing.
I can see why some people may tell you not to ruin the draw for others or w/e but really there have been many 3.5's who have joined "opens" and got eliminated in round 1.
That's the thing about being in an "open". You may be up against a solid 4.5 senior player, a 10 year old junior, or ... a random 3.5 who joined "just because".
I wouldnt join one, but I wouldnt tell anyone to not join one either as long as their intentions and expectations are in line.
"In the 1980's two men dominated--sometimes each other, most of the time everyone else."