xraf, I agree that orbits should not be necessary, provided that all aircraft fly a visual circuit and are flying at speeds that are more or less compatible.
But consider my home field, Rotterdam, which handles commercial traffic up to 737s as well. If I'm on downwind with a 737 on some-mile final (ILS), ATC is going to tell me to orbit. Because the alternatives are worse.
If he goes first (and he probably will, since he's on final and I'm not), I don't want to meet him, or his wake turbulence. So I'm going to have to wait it out, somewhere, for about two minutes. Continuing for two minutes on downwind takes me way outside the circuit, if not outside the CTR. If I just continue my circuit, base, final, over the runway, upwind and cross, all at circuit height, I'm making the 737 nervous because I might interfere with his go-around path. (And he might have to tangle with my wake turbulence!)
And if the tower lets me go first, well, the touchdown zones and exits on our runway are positioned so that even if you land long, even well beyond the light aircraft displaced touchdown zone, it still is quite a distance to roll before we get to the nearest exit. The aircraft I fly are used regularly for training (ATC knows the callsigns by heart) but ATC can't smell whether its a student or experienced pilot on board. They're not going to risk sending a 737 around because of a light aircraft taking a little longer to vacate than average.
So orbits in the circuit are a regular occurance where I fly. Nothing wrong with them if they happen on downwind or on any of the visual approach paths into the CTR.
Oh, and because of the interesting mix of comair and GA, first solos almost never happen at our field but go elsewhere.