It's not exactly a weak thing to thing as far as winning goes, it does help you win. (Especially in Turf Wars, I was against a few teams that would do this in the skate park and had devastating losses) Unless you mean a situation where your team hasn't inked up most of the field or they're moving towards the tower or RM, in which case it does seem more petty than actually contributing.
But, personally, I think it's a pretty immoral thing to do. If you're doing it in a TW where you've got The other team is getting almost no playing time, and your team isn't doing much themselves.You're pretty much just sucking all the fun out of the game just so you can win by a stronger percentage than usual. (I can't imagine spawn camping would fly with teammates in many ranked matches, though it might in some) And to get to that point, you have to be able to take care of the team the entire time you're advancing, so you ought to be capable of winning even if they were playing out a bit further. You don't have to make it close, just let them ink more of the area by their base at the very least. They'll get to actually play the game, and you'll have something to actually do. The game's meant to be enjoyed, not just to be won.
...I spent too long typing this and now I'm just echoing people's points, sorry.