Public Service Announcement: "SHA1(password+salt)" is an extremely unsafe way to store passwords. Use PBKDF2, bcrypt or scrypt.
Edit (T+7 minutes): Rewritten to not be a jerk.
Edit (T+7 minutes): Rewritten to not be a jerk.
Every little bit helps.
How To Safely Store A Password http://codahale.com/how-to-safely-store-a-password/
This is exactly why they are not particularly well suited to passwords. A brute-force attack (since you're salting your passwords I'm sure!) against them isn't a huge undertaking. Just with the equipment in the computer I'm on right now, I'm looking at generating about 11.5b MD5 hashes per second, or 3.1b SHA1 hashes per second.
At eight characters a-zA-Z0-9, I'm looking at about 5 and a half minutes to brute force every combination with MD5. Under a day for SHA1.
Hashes like bcrypt and scrypt, on the other hand, are designed to be slow. Their complexity factors actually provide means to slow the hashes down even further as hardware becomes faster. Instead of 11.5b/second, you can increase the complexity until you're only able to generate one hash per second... Now it takes you over three and a half centuries to hash what might take one second with MD5.
Even ignoring the possibilities of MD5/SHA1 being 'broken', they're simply too fast to be considered for hashing passwords.
(Estimates of GPU hashing speed taken from http://golubev.com/gpuest.htm).
Therefore 1) it is not useless to not increase your storage security even if Y% of your users use bad passwords as you are protecting 100%-Y% of you users. 2) Y% is probably not 99.8% for you, and if you are worried about it you can take steps to mitigate the problem.
ps. He is ignoring punctuation which is an important detail for actually doing the cracking.
pps. I appreciate the sentiment (users choose shitty passwords) but not the conclusion (so don't bother storing them well). The proper conclusion is use scrypt/bcrypt and increase the work factor. You can take reasonable steps to protect your users and you should.
Just to clarify: my original point wasn't that you should continue using md5. Rather, it was that bcrypt doesn't improve your security much. Given the problems with the 99.8% figure, it would be better to say, "the extra security that bcrypt provides might be less than you expect".