I use a vBulletin forum, which will only send me an update to a topic again if I visit the site - it is very annoying.
I subscribe to a topic, because I want to be told about all the updates - not only one which if I forget to visit the board for a short while serves no purpose to man nor beast.
IPS do provide and alternative for those who are probably in the minority and only want one update, but I am glad that they tell me about all replies.
I would actually say that sending one update to a topic is less likely to brig people back to a topic than all replies, as the first reply migt be a simple 'thanks', whereas the reply after that might be constructive or controversial and would prompt someone watching to return and join in the conversation.
Meant to say despite finding a way to add these, it would be nice if it was there as default as I could see these being very useful, especially when emailing lapsed members. It might save a few duplicate accounts.