There are at least a few reasons devs use Eclipse + Force.com IDE.
1) Many developers are used to using Eclipse for their development. The fact that SFDC has created an IDE for this environment proves this. For those devs that are programming in many different environments, it's a simple process to swtich between them. 2) You can't deploy from Developer Console. While Change Sets are getting better, they can still be a bit annoying, especially for devs pushing only code (triggers/classes/components/pages). 3) I don't know about any other devs, but I don't particularly like working out of a browser window when programming.
1) Many developers are used to using Eclipse for their development. The fact that SFDC has created an IDE for this environment proves this. For those devs that are programming in many different environments, it's a simple process to swtich between them.
2) You can't deploy from Developer Console. While Change Sets are getting better, they can still be a bit annoying, especially for devs pushing only code (triggers/classes/components/pages).
3) I don't know about any other devs, but I don't particularly like working out of a browser window when programming.