Most of the time I sit in Burmese posture. This is generally a more manageable posture for anyone with knee issues (and I certainly have my share). So when it's necessary, it's necessary.
More recently I've been training my body to adjust to half lotus. What's preferable about the lotus or half lotus position is that they're more stable and easier on the back. It takes less work on the part of the back musculature to keep yourself upright.
I highly recommend introducing your body to half lotus or if you can handle it, go for full lotus. Take care not to hurt yourself in the process but a slight amount of tightness in the knee at first isn't unusual as you adjust so give it some time.
Zen is primarily a physical practice and it isn't supposed to feel nice and easy all the time. Having said that, beware of the Zen Nazi's out there who claim that the only way to sit is in lotus or half lotus, and if you're not sitting that way you aren't truly doing Zen meditation.