Wouldn't it be easier to just update any existing add-on use Guzzle 5?
Creating one addon which provides Guzzle 6 support is arguably less work than modifying 10+ addons to work with Guzzle 5. But as
@NixFifty mentioned - I do have 3rd party packages which require Guzzle 6 creating the main driver here.
It's not as if Guzzle 6 is bleeding edge - 6.0 was released in May 2015, so it's well over 2 years old now.
If you use this and have add-ons that expect the version of Guzzle that comes with XenForo i'd think it would cause issues wouldn't it?
Yes, and that's the main gotcha that people will need to manage. However, once again - Guzzle 6 is the currently supported version and has been available for over 2 years now.
But that's also the case regardless of which version you use - managing dependencies is always tricky.
Given that Guzzle 5 and Guzzle 6 are fundamentally incompatible, you cannot mix libraries which depend on the other version.
But keep in mind that it's also more likely that people (like me!) will have already implemented XF 1.x addons to use Guzzle 6 because there was no dependency clash in previous versions of XF. So restricting XF 2.x to Guzzle 5 is actually a massive retrograde step from what we have now in XF 1.x
What benefit does Guzzle 6 have over 5?
If Guzzle 5 is working fine and you don't have external dependency requirements for Guzzle 6 - there's no real reason to upgrade and the version shipped with XF 1.5 will work fine. It's not as if it's broken or anything.
The main benefit to Guzzle 6 is that it is the version which is in active development and so will have improvements made which could help in some edge cases. There are a few new features which aren't available in Guzzle 5 - but for the typical simple use cases that XenForo integration would use, they are unlikely to be significant.
The other significant benefit moving forward (and the main reason for the incompatibilities with Guzzle 5), is the move to using
PSR-7 as the HTTP message interface. This change to a standards-based approach allows Guzzle to work with any other library that utilises PSR-7 message interfaces. This will allow for a lot more interoperabilty as more people start to implement PSR-7.
Again though, that's not likely to have much impact on most people developing addons for XenForo - at least not in the short term.
At the end of the day - I would urge caution before using this addon of mine - if you don't have a compelling reason to move to Guzzle 6, then stick with the natively supported version of Guzzle 5 for now as this will be the less risky option.
I mainly built the addon for my own benefit to support all of my own internal addons and libraries which already use Guzzle 6 - and as an exercise in learning how XF2 addon development works.