I have had a .45 of one or another type for 20 years now. The reason I "need" it (my justification) is momentum games. I originally bought one when I got into pin shooting. Now days, I don't do a lot of that sort of thing, pepper popper shoots now and then, but nothing rocks the steel over like a big, heavy bullet. Effectiveness in self defense? Well, I normally carry a 9 because it fits in a pocket and it has a good track record with 115 grain +P in defense scenarios. It's all I need for self defense and easier to carry than even the Taurus pictured above. If I go IWB, now days, it's the SP101 with 140 grain Speer bullets at 1340 or something near. It's as much as any +P .45 out of a lot longer, heavier gun. The little SP101 is a pocket cannon. Well, I carry it IWB because it's a might heavy for a pocket, but it rides light and speaks with authority. I rarely carry my .45 anymore, though it will conceal okay under a loose enough shirt IWB in the Sparks Summer Special I have for it. I shoot the thing as well or better (a Ruger P90) than anything I have and it's very accurate and 100 percent reliable. I shot IDPA with it for several years, fine weapon, I just like revolvers and compact pocket 9s for carry. Most .45 platforms ain't really of a CCW size and those that are ain't any better ballistically than my pocket 9, so I've not fallen for the .45 hype. It ain't the caliber or the gun anyway, it's the man behind it. I do like carrying a service caliber, though, as opposed to my .380. The only real worry I have about .380 caliber is adequate penetration. The service calibers, .38 special and up, I'm confident will penetrate.
Do you NEED a .45? Probably not if you're talking self defense, but that shouldn't be a reason not to buy one. I don't want to be without a .45, myself, if only because nothing quite flattens a pepper popper as fast and as sure. It does have ol' moe on its side. To be honest, though, the .40 shooting 200 grain stuff is about as good as the .45 even for momentum. I just never got into .40. When I got into .45, it was before the .40 was even thought about and the ten was an oddball curiosity in an oddball gun called a Bren.