I was debating between use and depend on,but I'll vote depend on for the simple reason that they make my life easier.
The main adds I use are auctioneer/enchantrix,koordinator,quartz,atlas maps,gatherer,flo aspect bar for my hunter,necrosis,and cryolysis, and Omen.I just recently started using questhelper.To some,that may seem like a lot,to others not very many.But I try to keep only the ones that actually make my gameplay easier.Auctioneer I could live without.I never go completely by the prices it suggests,but it does help.Koordinator is just too handy to do without.I like knowing exactly where I am at any given point.Quartz lets me cast spells almost simultaneously,or so it seems,since I'm seeing the actual lag between me and the server.Gatherer I could do without,because by now.I know where the herbs live pretty much by heart.Necrosis and cryolysis are another matter.
I have three level 80's.My main is my warlock.Without necrosis,managing my spells and buffs would be a royal pain.It's nice having one spot where I can make my various spellstones,and use them,plus having my demon/buff spells on handy buttons.Otherwise,I'd have to put them on action bars or open my spellbook.My spells at level 80 have all the action bar slots full already.Cryolysis is handy for my other alts,mainly for the mount button,tho I use the main sphere to cast the buffs I use most often for those alts.
Omen for my hunter and warlock is also an addon I suppose I could live without,but wouldn't want to.I know the Blizzard ui warns you,but I'd rather see the bar,instead of getting a gaining threat message,followed by "Attacking You!"
Being on the PTR made my vote easier.I keep most or all my adds turned off,too many headaches.But I have found that doing without them makes the game less enjoyable for me.I spend too much time trying to do things that became intuitive with my addons,and it's gotten me killed a few times.
So,in the long run,I COULD live without any addons,but I wouldn't wan to.For one thing,it would remind me too much of FFXI,where if you use a single addon,you're account is cancelled and you are banned for lfe.If Blizz ever went that way,I'd probably kiss wow goodbye,not so much for the addons,as on general principle.