I am definitly more of a collector than a player, say 75 -25.
The reason is simple, with a job, wife and a life outside magic, the time for gaming is very limited.
90% of my time with magic is spent at home alone, and thinking about decks or reopganising my collection.
Why does the limited time to play made a collector out of me ?
1) The lack of playing practice makes for a general lack of knowledge of the cards, esp. of the new sets.
Collecting hte entire sets helps greatly becoming familiar with the cards without actually playing. And If I am looking for cards to put in a deck, I simply browse throught the sets (neatly arranged by blocks in individual binders).
So the lack of playing practcie made me a set collector.
2) PLaying little means that you cannot always be on the top of the wave : having the latest tech in Bloc format or even t2.
It is easier to have a few powerfull decks for extended, 1.5 or type 1 to upgrade with a few selected new cards. The collector will be ready to build any "proven archetype" thanks to its pool of good cards.
3) Being ready : when you feel like building a deck, you want to have the cards available RIGHT NOW, and you dont have the chance to go trade every day. So you trade "in advance" just in case you might need the cards. (e.g. I got 4 tithe even though I have no deck for them yet)
Since there is no real focus into deck building the collector, as you said will tend to get 4 of the "good rare" - just in case you suddenly feel like changing your cuirrent deck -
4) collecting "older cards" is easier than getting the new ones (not true for the "very old ones, but at least for the extended scene).
Why ?
a) proven value. Everybody knows which are the "real good cards" of older blocks. not much you can do wrong by having 4 morphlings. The "hiped" cards are also not necesserily the real good ones for the long term (eg Orim's chant)
b) cheapness & timing. Try to get your hands on absorbs or vindicate. When the cards are in bloc format or T2, there value tend to be ridiculously high. For 12 USD I prefer having 3 tradewind riders that a Vindicate, thank you very much.
c) Bloc rotation. The event every collector is waiting for. When a bloc rotates out, a great opportunity to get for cheap those power cards that are no more T2 legal.
5) Respect & Pride. That one might sound stupid, but if you do not have the time and ressources to become a "great player" with the best current bloc deck, at least you can gain the respect of your peers by the volume of your collection.
On a side note, this is why magic is great, even if you cannot play, you can still be active in the game, through the colelcting aspect.
0ne last note, I sincerely believe that you can build a great collection of even older cards if you are an astute collector.
Since I move from competitive play to a more "collecting" attitude, I actually spend less money, but increase the value of my collection.
As an example, without ever spending more that 20 USD cash per card and through good trading, I got myself over the last year :
4 Force of will
1 Library of Alexandria
1 Mox Saphire
1 Timetwister
1 Moat
1 Berserk
1 Tabernacle at pendrell vale
enough "patting on my own shoulders", time to log off ..