I guess this is where I differ in opinion. I like when gifts are opened. 1. I want my child to learn that not all gifts are theirs and they need to be patient and watch others.
2. Part of the joy of buying gifts is watching the child's reaction...good or bad. No hurt feelings if they don't like it. If I knew they weren't going to open is just buy a gift card. I hate picking and wrapping gifts.
3. If you run out of time I understand that. (ie bounce house, etc)
This is where I'm at. I think it's rude not to open presents. Along those same lines, I think children should only be allowed to invite however many children as years they are turning. So yeah a 4 year old opening 15 gifts is ridiculous but maybe 15 friends is just way too many to invite. A 7 year old can sit through a friend opening 7 gifts. As can a 10 year old with 10 gifts and so on.
I usually invite the whole class which is the local custom here (so 16 kids for DS and 22 for DD). We also add on close friends from outside class if we're doing the party at a bounce place since they don't really have a size cap.. If this rule of thumb works for your family then great but I don't think it is at all a common social convention. You're being narrow minded if expect other people to follow it. I'm all about people doing what works for them and small parties work for some but for all families.