They're not really "replacements" per se. The other apps is just a new "skin" on the stock app. The basic contacts data is still stored in your stock app.
I have my work contacts on my phone...which is over 700 names, and oftentimes, I can remember a company name before I remember the contact's name, especially if it's someone I don't talk to frequently. So, I find Contact Groups extremely useful because it lets you view contacts easily by Contact's company, title, and groups (syncs to the groups you set up in Google, which the stock app doesn't do)...And using these groups, you can a group SMS or email easily.