I'm very picky over DLC and basically only get those which cater to my interests. Therefore I do think they are necessary for me to really enjoy the game. E.g. Seasons is a must, something any "life simulator" should have IMO. I also really want my vampires and other ways to spice up the core gameplay. They can also be important for mods since the first gameplay mods I download tend to be vampire related, mostly stuff that results in, ahem, "feeding accidents". Any vacation type of packs (i.e. ones that are focused on specific location) I tend to skip. I also don't care about building focused packs because I'm in it for the gameplay and the sims themselves.
The importance of DLC had some differences between Sims tittles though. TS2 basegame was a very fresh experience because of being such a huge improvement over TS1 (for which I never got any DLC) and thus it kept me entertained for a loooong time. Eventually, I did get Seasons and then some other Eps. TS3 I got bundled with Late Night and, coupled with the new open world system, it was another rich experience from the start and kept me entertained for a long while but, again, I ended up buying Seasons... and then some other stuff. With TS4... I grew bored of the base game in just one day. Since there happened to be a sale, I went and got some DLC: Seasons (surprise!), Vampires and Parenthood. During later sales I got some more stuff relevant to my interests. Because TS4 lacked a "new hook", it's the one TS tittle in particular that really needed DLC in order for me to enjoy it.
I guess some could say "well played EA" since they certainly got me to buy DLC fast :|. Oh well, got the base game for free and the DLC from a sale so I have no regrets.