I think you extremely underestimate te time invested in makeing a littlecup team. I think it is a fair point that gimmicky tournaments (like littlecup or the fight before christmas like today) have signups longtime in advance so you know that you are ensured a spot, because it would really suck to have spent multiple hours doing hte last few weeks to make a team and then not even make it into the bracket... I don't see this signupmethod working for gimmick-tournaments like that.
That being said, I see no problem in this signupmetod for standardtournaments like OU/UU/NU, in general people have comps for these allready. Some might want to make a new team specifically for these but it is not a must like in the gmmicks and you can always use the comp in the next tournament, unlike the tournaments that only come once a year. Apart from the few minutes gained due to the rolecall being shorter that are easily overshadowed by the fact that you have to be there an hour before, I think this signup method could help those who would else not have ome due to being low down on the reserve. I think 5 of the last 6 tournaments I've showed up to I signed up in last minute and still got in even though the list of reserves was long. Some of this is probably because people, like Jonas said, just signup without knowing wether they will actually show, and some of the reserves not coming because they are so far down on the list that they don't tink that they will make it into the bracket. I think this signup method might help (not at all sure) but I think it's a good idea to test it again, but it dosn't fit for gimmicktournaments where you make a specific team just for that single tournament.