Why do you think not wasting resources introducing legislation that’s guaranteed to fail is “rolling over and pissing themselves like whipped puppies”?

It's very important to demonstrate your party's values by introducing and passing bills that reflect those values so that you have a record of where you stand that people can see, regardless of whether those bills get vetoed or not. Saying you should wait until you have full power, or assuming they are "guaranteed to fail" is self-fulfilling prophecy and is capitulating in advance.

1. It demonstrates exactly what the Dem party's priorities are and what they will do when they have power. People will clearly understand what they will get if they elect a Democratic president and congress.
2. People will clearly understand what the Repub party's priorities are when they vote NO on bills that they personally want passed, and when a repub POTUS vetoes them.
3. The Dem party will receive positive feedback from constituents when they pass bills that constituents want, and that will make them more likely to continue passing even more bills that people want.
4. It gets publicity and (if popular) builds more public support, which puts more pressure on both reluctant "moderate" Dems and even the Repubs, to get on board with voting for popular legislation if they realize they are more likely to get re-elected when their constituents like what they're doing, but less likely to get re-elected when a lot of people see them kowtowing to their donors and voting NO on popular legislation.