I was a cast member in MK attractions for about 3.5 years (2002-2005), and saw a lot of instances of "cutting". Some I let go, some I stepped in and stopped. Disney's "offical" policy is pretty lenient, but each cast member usually enforces it to a different degree (more often than not, stricter).
On most rides 1 person joining their party (or one person and a *small* child) will not effect when *I* get on the ride. On many rides, parties are sat together usually in thier own ride vehicle (i.e. dumbo, magic carpets, winnie the pooh ride, etc...), so whether they have 3 people or 4 it won't matter, it'll take the same number of ride vehicles.
I'm apparently in the minority because I have no problem with the "hail mary" (I think that's what it's called...sorry if I've got that wrong...I've never heard that before...) pass. If it is only 1 *young* child being passed to another parent. PLEASE don't flame me for this, it's just my opinion (and, no I don't have kids. It doesn't bother me (nor most cast members) because it actually makes no difference to the wait time. (If it's 1 young child, and their 4 brothers and sisters, aunts, uncles, etc... then I have a problem with it)
As far as adults "cutting".... If it's *one* polite adult catching up with their family, that they had to leave for a particular reason, that's ok. (I wanted to shop...is not a good reason).
If one person gets in line and then 20 minutes later their 14 friends try and join them - that's not ok. (I once actually stopped a family who did this (2 adults waited in line and 13 yes
thirteen others (kids & adults) tried to join them about 85% of the way through the *long* line...yes I got called a lot of special names by those people...and then security came...I got called a racist *bad name for a girl* and then the mean people went away).
I'm usually pretty easy going at the parks, and as long as you are POLITE I will let you and your young child catch up with your family. Because in my experience it won't really make a difference in how long I wait. (Also I do not want to wait around a screaming/smelly child)
Also, as far as the Guest Assistance Passes are concerened, (when I worked there) there were only about 2 reasons a guest could get a "front of the line/no wait" pass. (1 of which were make a wish kids), other passes either got the group (up ot 6 people) free fast passes, or a trip through the regular que with everybody else. (in some instances we allowed the person who couldn't wait in line, with maybe (depending on age/situation) one other person wait outside of the ride que, but close by and they were allowed to board the ride when the rest of the party made it to the front of the line (by waiting with everyone else).
Most ride que's are fully wheelchair accessible, so not waiting in line because you can't stand for a long period of time, shouldn't be an issue. But if you have a condition such as IBS or something that causes frequent dire need for a restroom, you can't get a front of the line pass for that, nor can you get a fast pass on all rides, so if the person needs to get out of line run to the rest room, come back and join their family, thats fine. I don't want anyone to be in pain while waiting for a ride. Especially when they don't have to be in pain. (Nor do I want that person to have to expain their, often embarssing, medical condition to everyone (or anyone) in line.)
Sorry if I kinda rambled...

Hope my point came across though.