Just keep them and wait. There may come a time in the future when transaction fees become smaller, and value is higher. Perhaps someday in the future the 0.1 mBTC will be worth $100 and the fees to spend it will be only 0.01 mBTC ($10) so you can consolidate it and still keep $90 of value.
Thanks Danny,
Yes, I guess this issue happens to anyone. So I am wondering why there is no solution for this, except hoping for the miners to take tiny amount of fees.
For me, I don't have enough amount of BTC anymore on a single address to buy something I need. Those small amount of BTCs are mostly the remaining BTC from my previous transactions in which I always use maximum 2 addresses to reduce the fee. Well... I guess I have to start mining again to get more BTC into my wallet.