I don't think it's a big deal if it's one child, maybe even 2. However if 1 person stands in line and there are more than 2 kids who come at the last minute and they each want a seperate photo, I don't think it is right. We didn't realize prior to our first cruise that you had to get in line before the stated time as often they would have the lines closed already. When we finally realized this we would get in line early. One night we figured we had plenty of time, we ended up being half an hour late for dinner. But after spending well over an hour in line I wasn't getting out of line. We go a great pics of our daughter all dressed up with Mickey and Minnie so it was well worth it.
They base when to cut the line off based on how long the line is. I'm not sure what would happen if they cut the line off and it's time for the character to go in, since I'm not sure what the rules are on the cruise, if they will stay until the whole line is finished. I know that nothing is worse than standing in line for an hour and having a ton of people jump in front of you. One or two is acceptable, more than than in my opinion is not.
Besides which I feel that waiting in line is an important lesson to learn. We bought my daughter a notebook/calander datebook type thing a couple of years ago. I put twistable type pencil/crayons in it and gave it to her. It has a strap on it, so she would take it with her and when she got bored in line, she'd open it up and draw. When waiting line for characters or other things I show her how long the line is and ask her if she really wants to wait. If do give her wiggle room, etc. On the cruise we one person would walk around a bit with her and check back in, but at DL/WDW I don't do this, and didn't want her to get too used to being able to get into line the last minute.
Of course she's been going to DL since she was a baby so she's had a lot of practice waiting in line these last 5 years.