Glad to hear that you are a brass player. You say you're amazed that I don't "know" this - sorry, but when I play legato intervals, I am able to do it without hitting the partials in between. I don't know why you're hearing something I'm not, maybe your horn playing needs some work, or maybe horn players just have a lower standard in this respect? I will readily admit that horn is by far the hardest of the brass instruments. Again, I understand your explanation about the "slow legato" but I simply don't agree that playing over an interval slowly enough to hit the partials in between would be considered "legato" and I don't think most brass players would either.
No question that trombone glisses are a special case. Does VSL (or any sample based library) have a way of covering that aspect of playing with any sort of flexibility? That certainly does seem like a situation where physical modeling could probably do a better job of getting the playability.
When I get a chance, I'll record some intervals and post them so you can hear that they can be done without hearing the partials in between.
Anyway, back on topic, if you want legatos to have some "slop" of the in-between partials, there's no reason that can't be included in the modeling as well.
Oh, and a trombone can only gliss a maximum interval of a tritone, the slide isn't long enough for a fifth - if you actually do play a brass instrument, i am amazed you don't know this. [;)]