You don't have to go too far to find parallels to the Nats situation. Most teams aren't in the Yankees / Sox (more Yankees than Sox though, am I right? Stupid Red Sox!) position of having a playoff position feel guaranteed at year's start. They have a good idea that they could compete for a playoff/division spot but one or two more players would certainly help. Since nearly ever team has a couple guys in AA and AAA doing well, the question is forever in baseball: do you go for it with a big signing, or do you see if your own young guys can rise to the occasion. Last year we saw things fail both ways.
For those that don't want the Nats to commit to a long term deal for Fielder, there's the Chicago White Sox. Despite being one of the more successful regular-season teams of the past 15 years, the White Sox have more often than not been also-rans finishing around .500 and out of the playoffs. 2010 was no different, with good but not great pitching and fair hitting leading to a 88 win season and nothing more. So the White Sox made a bold move, signing Adam Dunn (you may remember him) to help bolster the offense and put the Sox over the top. Instead they crashed. Their young 2nd baseman regressed, their young hitting prospect did not do well in his first time up, their OF with possibly questionably good offensive numbers the year before cratered, and most recognizably their big signing of a big guy with a big swing came crashing down as he has possibly the worst season in the league. Can anyone see this happening to the Nats?
For those that want the Nats to sign Prince, there's the San Francisco Giants. A team who's success was predicated on the awesomeness of their young pitching had seen 2010 be "that year" where everything went right for them and they came home with a WS title. However, there was a big problem with the team as a mediocre at best offense seemed to be keeping the team from becoming the dominant force that the pitching would suggest it could be. They really needed another bat. However money concerns kept them from moving forward. They had made a couple of bad deals in the recent past that tied up money in fading veterans, and were very concerned about signing their young guys a few years down the road. So they went with what they had hoping that it would be enough. Unfortunately their best offensive player got injured and missed most of the season, the guying who played first base the season before with the oddly good year regressed, no young batter developed to be anything special, and their veteran signings ended up doing nothing. Despite the great pitching again by the young staff, they couldn't compensate for a terrible offense. Can anyone see this happening to the Nats?
There's not much of a point other than success and failure for most teams on that 85-92 win edge rises and falls with injuries and unexpected performances, good and bad. If you think signing Prince will guarantee the Nats a spot in the playoffs, it won't. He's not that good. If you think not signing Prince will help the Nats start a multi-year title run down the road, it almost certainly won't. There are too many things that have to go right for the Nats for this to happen.
Now if you think signing Prince AND signing or trading for someone else will virtually guarantee the Nats that spot and continuing this level of spending will ensure the Nats a multi-year title run down the road then you are probably right. Unfortunately, you don't own the Nats and some people didn't buy this team thinking it would cost them the gold toilet fixtures on their second yacht.