Yes, it's rude not to open, to say thank you, etc. It's an important skill for children to give and not expect to receive.
In my opinion, parties have become less and less personal over the years. People invite way too many guests, particularly for young children. 17 toddlers, the entire kindergarten or preschool class, 25 cousins, and so on - it's excessive. It costs a fortune. People can't host them in their homes anymore so they choose an expensive location - adding to the cost and the impersonality. It's almost like a contest to see who can put on the best "show" - there's a lot of pressure to keep up with the Joneses, and it's out of hand. Once you have a huge group, it's impossible to sit down and open 20 gifts, say thank you, and expect the children who are watching to enjoy this spectacle. It's repetitive, ostentatious, and boring. It's unreasonable to expect a bunch of 5 year olds to be thrilled watching someone get 20 gifts, none of which they can play with. Host parents feel guilty, so they go broke buying expensive goody bags, and they move to open the gifts at home.
My solution is to make a reasonable party. Don't invite the entire class - those parents don't want to go to 20 parties throughout the year, nor do they want to buy 20 gifts for kids their child hardly plays with. They don't want to spend every weekend shlepping their child to yet another venue and feeling pressured to be part of this particular boost to the toy-and-party-place economy.
Follow the "year" rule -- if your child is 4, he/she invites 4 guests, opens 4 gifts, says thank you 4 times. If your child is 7, invite 7 kids. By the time the child is 10, he/she may want to do something more expensive like go to a movie & lunch, and then you cut the guest list down to the number of kids who will fit in your car - which is usually 4 or 5. Children learn to choose friends who will enjoy each other. That doesn't mean the birthday child can't have other friends or play with them at recess! It just means it's a small group. Play a few classic party games (the kids think they are brand new because no one plays them!), do a quick craft or have a treasure hunt (and the craft/treasure becomes the goody bag favor), eat some cake, and run around.
It gets our kids out of the materialism thing, gets back to actually celebrating a birthday instead of watching the parents go broke, and the guests get to actually feel special and appreciated, both from getting an invitation and from watching their gift be opened and enjoyed. They learn to enjoy the act of selecting a gift, and of being thanked. The birthday child learns manners (expressing gratitude even if it's not a gift he would have chosen for himself), and actually learns to write a thank you note without it having to be a tedious act because he has 30 to write. We have a whole generation of college kids now who don't know how to have manners in a job interview or to write a thank you note to a mentor - and we wonder why we have selfish, me-me-ME young people! It starts when they are young - and it's our obligation to teach them!