You should've asked your dentist to explain it to you. Having said that,
that tooth is probably not evidently decayed in the X-ray (you never know
clinically). Otherwise your dentist would've surely made a new filling for
you. S/He is suggesting a crown probably because the filling is huge. Your
remaining tooth structure may crack under biting forces and then even a root
canal might not be able to save your tooth. I would follow your dentist's
recommendation.

Maybe.
Or you may fracture the tooth rendering it non-restorable.

Get a second opinion.

Sounds kinda steep for a crown, but remember that dental
fees vary widely with regard to geographic location.
--

W_B
Take out the G'RBAGE
wubbabubbazG@RBAGEyahoo.com

I'm not a dentist, so maybe my explaination will be in simpler terms.

A molar tooth is kind of wedge shaped, with the top being the flat end of
the wedge.

A filling (can) be like putting another wedge within a wedge. Chewing with
the tooth can put splitting force into the tooth.

If you chew on something, and the filling drives in, it may split the tooth.
A vertical crack which goes down below the gum line. And, that's not
something a dentist can refill.

A crown goes over the top, so that chewing pressure is straight down. Not
pressure that wedges into the tooth and tries to split the tooth.

Hope that is of some help. In the meantime, please try not to chew anything
over that tooth. Well, not anything which requires any kind of chewing
pressure.

A root canal removes the nerve from the tooth. Root canal won't mend a split
tooth.

I've been putting off getting a crown on a molar. Although at each
checkup, my dentist says that the filling is weak and I should get a
crown.  I am probably under the false impression that if it doesn't
cause me pain I can wait. I don't look forward to the procedure or
shelling out the 12-1300 bucks. If I just wait, will I risk having to
get root canal?