It's like a rumble pak you can put in the gba slot.  it is pretty cheap on ebay.  i don't know what games it enhances.

It's not essential.

check out the wiki link I posted.

I think a lot of games in general benefit from rumble. it is fun to fill the recoil when you shoot a gun or your blade makes contact with an enemy for example. However, I think only a few types of games require it in order to be playable like stealth action games. I think stealth games are unplayable without rumble because often it alerts you to enemies and their proximity to your location.

@OP
are you seriously comparing the difference of double the CPU power(apx 120mhz now), 3 times the ram(16mb instead of 4mb). Better firmware that is upgradeable, DS Ware content, 2 digital 0.3mp cameras, slightly bigger screens, unfortunately DS Phat like battery power, sd card slot for music(blah aac), WiFI that supports WAP instead of just WEP(or am I wrong). Just for the option of having a to buy a GBA Rumble pack for a few games?

In comparison I cannot fathom even if the DS Lite had internal rumble and you never went online with the DSi that you would exchange the other features for a poor micro motor rumble.

      Well, I don't have a wireless modem, so I can't go online with a DSi if I had one.  The big thing that is keeping me from getting a DSi or DS Lite is the $40.00 price difference.  $169 is a bit steep to me, personally.  But the DS Lite since it is lacking all those features you mention on DSi is probably a bit steep too since it only costs $40.00 less.  I wish they would bring DS Lite down to $89.99 and the DSi down to $149.99 (because it's not even $100.00 cheaper than the Wii as it is now).  I like rumble and knew it was a feature not in DSi, but from what I've read about it like from Nordlead's post with the wiki link rumble isn't essential to ds games.  It seems like the biggest game difference between ds lite and dsi is you can't play guitar hero world tour on dsi, but I've never played any of the games in that series.

damn the dsi doesnt have rumble now i wont be getting one

Thanks!

The DS doesn't have rumble standard so I think it's safe to assume no, rumble isn't essential to the DS experience.