Not really any noticeable downsides. It'll produce a little more heat and have a little impact on your battery life, but unless you sit there with lab equipment and note things to the nanosecond, you shouldn't see any downsides. Other than financial as noted above, and it'll depreciate more so it's not like you'd get the cost back if you plan to sell it. If you need it then get it, if you can't work without it then it's a silly suggestion not to get it.