The two are so close together, you can just walk the distance. Would not matter much if it was destroyed. It brings in people from NCR.
Still I could be wrong on this. People talk about how troops come from NCR using the monorail.
Yes and no. It does matter if the Monorail Line is blown up, the Monorail is a 2 minute ride safely from McCarran Airport to the Las Vegas Boulevard Station. If NCR goes through Freeside, where alot of NCR resent is, there could be trouble, yes a trooper could kill a local who picks a fight, but if some other local is just strolling by to see that trooper kill the guy, they'll go around telling people that he saw an NCR guy mercilessly kill a local. So mainly for diplomacy, it's better NCR is kept out of Freeside, I mean hell, look what Dixon is doing to those two guys just because they are FROM NCR.
On an offnote, only NCR troops or NCR VIPs are allowed on it (IE The Courier with a Liked status with NCR)