maybe not "have" to buy in fractions but i think if the price ever goes up to $10k -100k per coin then the average user wiill not be buying full btcs due to the price
a mbits system would be more appealing for new adopters too than a tiny fraction of a whole coin imo
its just psychological

We are already there, posts on this site frequently mention getting in as an investor at less than a full BTC. I do think that moving trading platforms to quote in fractions of BTC is a smart marketing move, but not necessary.
I think BTC can easily pass the market cap of large cap stocks in 2014 (500 billion). This is the bubble investment opportunity of the last 1000 years. I seriously cringe that i didn't jump in two years ago when i read about it, but that is what it is.
My main concerns are alt coins, rich developers getting lazy and not progressing the protocol, government intervention.