I'd like to see us give this a decent shot and play a strong team.
Yeah- it's only a friendly- but it's a friendly with bragging rights- and a plastic trophy awarded. Overall though... more important than any of that is momentum.
There are pretty thin margins between clubs at the top of the league table. 50% of the difference between 1st and 7th is tactics/ability- the other 50% is confidence/momentum/morale and motivation. Entering the Premier League off beating Manchester United in a *ahem* cup final and a 4 match unbeaten run against big European clubs (granted in friendlies) will be a big morale boost.
Conversely, losing will give United that extra early season boost- coupled with "New Manager" syndrome where players play extra well for first few games to impress new manager.
Beating United tonight will give us an advantage in PL and block a rival gaining an advantage... it's important.