Yeah, now I think of it, you're right. I thinkk you can only summon something you yourself would have the strength to carry. Also, I'm sure the distance is an issue too, like how far you could carry the object.
Haha, my good friend went to Canada for holiday and I was trying to accio him.
Needless to say, it didn't work.
Quite simply put, if you could Accio someone, then why didn't the Ministry have several Aurors jointly Accio Sirius Black during PoA or OotP? For that matter, why didn't Voldemort and company Accio Harry during HBP or DH?
That's what I thought too but I was thinking someone close by like in the same room or within a few yards of each other.
I believe someone summonend their friend either at the Yule Ball or Slughorns party, but I'm not sure whether he was actually using accio or not.
Oooh, interesting question.
Some people have already pointed out the problems they can see with it so my advice is that if you need to have someone Accio someone else then give it a twist. For example, what if they try it and then end up propelling themselves towards that other person, or something like that. (I'm sure you can think of a more interesting outcome). But yeah, I'm big on magic doing things it's not supposed to in extreme situations. I've had someone "accidentally" go back in time among other things and the readers didn't seem to mind, either. You just have to write it in a way that it doesn't seem too convenient, in my opinion.
/too many cents
Kerichi makes very interesting use of a modified Summoning charm used in terms of people in her story More Than a Feeling. It starts out with Severus experimenting and has interesting consequences. I think it is rather elegantly done. You could have a look here, for curiosity's sake:
Chapter 10 - Summons
EDIT: If the discussion must stay purely within the canon framework, I give my humble apologies and am ready to remove the post.