If you're using DI without an IOC container, the biggest downside is you quickly see how many dependencies your code actually has and how tightly coupled everything really is. ("But I thought it was a good design!") The natural progression is to move towards an IOC container which can take a little bit of time to learn and implement (not nearly as bad as the WPF learning curve, but it's not free either). The final downside is some developers will begin to write honest to goodness unit tests and it will take them time to figure it out. Devs who could previously crank something out in half a day will suddenly spend two days trying to figure out how to mock all of their dependencies.
Similar to Mark Seemann's answer, the bottom line is that you spend time becoming a better developer rather than hacking bits of code together and tossing it out the door/into production. Which would your business rather have? Only you can answer that.
