This kid must be so confused by now
Reading through this thread, the OP can't be any more enlightened than before he put his post on RB.
So in keeping with the theme, I'll add my two cents:
If you are going to do this logically, ie you really want a single bike that can handle both DH and a bit of general riding around, including up hills, you should follow the advice of several people who have already posted, and get a 6" AM style bike such as a Giant Reign or Specialized SX Trail. These bikes can be very competitive in the right hands at DH races, great for a beginner DH rider, and a lot better for general riding around, including up hills, than a full DH rig. The reasons for this are thoroughly explained by thecat above, and note that this confirms that your Dad is correct.
However, if you have other reasons for wanting a DH, eg you want to look cool going to the shops, you want to make your XC loop twice as difficult for extra training, you could get a DH bike, and make a couple of minor changes to make it easier to ride, when you are not doing DH. These changes, in order of priority, are:
1. Replace the DH tyres with slicks. Here's some slicks for $9 each, my daughter has them on her "around town" bike:
http://www.torpedo7.com.au/products/INTYMNNPP/title/innova-mtb-slick-tyre---puncture-proof. These will roll a LOT better than DH tyres.
2. Fit a XC cassette. Lower end/older DH bikes often have them anyway, but if your DH bike has a roadie cassette, replace it with a 12/34, it will make climbing a lot easier.
3. Fit a long stem eg 120mm, this will give you more cockpit room, and allow you to get more over the front of the bike on climbs.
You really need to consider how much time you will be doing DH on the bike, vs other general running around. Yes, you can ride a DH bike around, but as others have said, they really do suck at anything apart from DH.