We seem to have oodles of spare space currently
The problem is the rate of adoption likely isn't linear but exponential meaning it just seems we have "oodles" of spare space. This could change quite fast.
Competing centralised services built atop of Bitcoin to cater for micro payments will not only be more efficient and cheaper ...
These services do not exist yet. It is the root of the problem because the 1Mb limit will cripple bitcoin (later this year) before such services can take over micro-transactions (several years away). The limit needs to be flexible enough to allow bitcoin to work as it does now, until/if synergistic competing services develop on their own merits.
They don't? Users of mtgox can send each other bitcoins off blockchain, so can users of most other exchanges, hell Coinbase is even built with this being it's primary goal..