hey there!
i think its a way for them to expand the market. i myself have not tried purchasing it this way and u might lose some of the bonuses like free months etc..
also, if you don't use my link you don't support our project ;-).
but if you don't care about all that... well you get 2 years for $89 versus my link gives 28 months for $105.
Conclusion:
2 years for $89 is slightly cheaper at $3.71/month
my link (28 months for $105) is $3.75/month, which is very close but a bit more expensive.