Has anyone

[ZiNgA BuRgA]

PayPal offers a simple code for donations, which anyone knowing how the template system works, can easily embed onto the forum, so the only issue here is paid subscriptions.

Well, none of the forums I regularly, or even, irregularly, visit have paid subscriptions of any sort, though I guess this feature may be used more in specific types of forums (but not really forums in general).  Most forums are really a discussion platform, rather than something involving payments.
Then again, MyBB has a number of things which don't get used too often in general anyway.  I think they mostly implement stuff which are relatively not too difficult, and stuff requested by many people.  I haven't seen that many requests for a paid subscription feature, so they probably decided not to bother with it.  Another thing could be its reliance on third parties to handle things correctly (ie must interface with Paypal) which isn't exactly a nice thing.

As for modifications, I personally don't have any interest as I don't charge people for stuff, so I haven't bothered making one.  I suppose many other authors feel that way too, especially if they're releasing stuff for free.
That's my guess to your issue anyway... >_>