Actually iPhone understands such situation, therefore the company provides an option to answer incoming calls with speaker. That option can be set by navigating to Settings > Accessibility -> Incoming Calls, and there are options available to be selected which are Default, Headset and Speaker.
This feature can also be utilized by defected people, beside to those who are described in the first paragraph. However there is a problem that you might dislike, because you get to make a repetitive press the speaker button each time there is an incoming call, while keeping your hands free.
This tweak is free and currently available and listed under BigBoss repository for free. To make it enable, simply choose the more option in SBSettings pane, then select the Set Windows Toggles option to get access into the iOS 5+ Notification center pane or the Dropdown Windows, and set this tweak to on. And if you want to switch back to normal behavior simply turn this tweak to OFF.
How about icon, will Speaker SBSettings Toggle adds icon to home screen? Don't worry, there will be no icon added to your iPhone's springboard.
Interested with Speaker SBSettings Toggle package? Head to Cydia and then grab it under BigBoss repository to install Speaker SBSettings Toggle for free of charge. However tell me what you think about this package, does it serve you well?