Collecting just a sub-theme

Has anybody ever considered collecting just a sub-theme such as just aircraft in the city theme or just cars in the Technic theme? I am considering selling off some of my Technic and just collecting the earth moving / construction / truck type sets within this theme. Would doing this limiting me too much? I am also looking at this to focus my MOC building in this direction with MOC’s which are related to this area.

Looking on Brickset I could include 20 sets in the last 5 years within this theme and eBay has quite a few sets which would fit in this area, some quite old and rare.

Do you collect a wide range of themes and sub-themes or do you limit yourself to just one?

I limit myself to just stuff that's useful to build my train layout. That ends up being a mix of city sets, train sets, modular houses, some licensed stuff for parts and a heap of minifigs.

I also have a collection of stuff for nostalgia reasons which tend to be stuff I wanted but couldn't have as a kid. and then there are a few really cool sets I just really like.

I can't say I've ever wanted to collect just a single sub-theme.

I like to collect a variety of sets.
I haven't sold any of my sets and I don't plan to.

Thanks for the replies, I always collect based on what I like (which does sometimes change over time) however my question was more based around if other people also do it and if so how do you decide what to base it on.
I have decided to head in the direction of the construction / earth moving / trucks theme so this will focus my collection and give me something to build up.

Im not a collector just a MOC City builder, I am worried that one day I will be a collector and this scares me as thats when you really look for and want to buy MISB and they can cost so much $$$$$. But in saying that you would own a decent investment too, Its not like Lego prices will drop.