Well yes, some people do, but if you're dedicated to playing a particular army (like us Space Wolves players!) then we tend to need to take a long term approach to what be collect, build and paint.
Remember back when Las/Plas Razorbacks were the new hotness? Nowadays, it's all about the Drop Pods.
An Army Made of 5 Armies
The point I wanted to make is that I'm seeing some tournament armies appearing which are made up of various allied detachments and formations bolted together to create a great army. But, my concern is that as soon as 1 of these factions has a rules update or a formation is changed, then an entire army (and most of someone's collection) becomes unplayable.
For example, this Adeptus Mechanicus themed army by Geoff Robinson contains
But most importantly, it uses the Flesh Tearers detachment from the Blood Angels Codex to allow it to deploy via Drop Pod.
Should the Blood Angels Codex receive an update and this formation is removed from the rules, there's suddenly a whole army of miniatures that need an alternative mode of transport.
Simply put, the more allies and special formations you have in your regular army list, the more you expose yourself to changes. Not just from rules updates, but updates to the Meta also.
Taking 1 Codex and 1 Detachment
Call me boring, but this is one of the reasons why I chose to create my Space Wolves Drop Pod Army using a single Detachment, because it's less likely to be impacted by changes until there is a new Space Wolves Codex.
I was extra boring and chose a standard Combined Army Detachment with plenty of Troops to make the most of the Objective Secured rules for the Detachment, rather than simply trying to kill stuff.
But, while my army may not be top of the Meta (and if it was, it might be top for 2-3 months at best), it consistently plays well and competes will in an ever changing environment.
You would have to be extremely dedicated financially and in free time to try and keep up with the rapidly changing Meta right now. So rather than try, I'm suggesting that it may be better to create something that you know will be consistent and stick to it in order to ride out of the storm of new releases.