Complex logic on a ElementCriteriaModel parameter? How would you combine multiple logical expressions on a ElementCriteriaModel parameter?
You could query twice and then merge the two returned arrays of entry models,
but that probably wouldn't allow me to later use the {% paginate %} tag or methods like getNext().

Here's an example where I'd like to filter events starting in calender week 32 or 34:

{% set dateParamCw32 = 'and, &gt;= 2014-08-04 00:00:00, &lt; 2014-08-10 23:59:59' %}
{% set dateParamCw34 = 'and, &gt;= 2014-08-18 00:00:00, &lt; 2014-08-24 23:59:59' %}

{% set startDateParam = 'date range cw32 OR cw34' %}

{% set entries = craft.entries.section('events').startDate(startDateParam) %}
For most cases you can find a workaround to combine such conditions by making use of entry IDs.

In this example query for each time range condition separately and use the ids() method to "collect" your entries. All matching IDs are then passed to the id parameter of a final ElementCriteriaModel, that you can also use for Pagination etc.:

{% set dateParamCw32 = 'and, &gt;= 2014-08-04 00:00:00, &lt; 2014-08-10 23:59:59' %}
{% set dateParamCw34 = 'and, &gt;= 2014-08-18 00:00:00, &lt; 2014-08-24 23:59:59' %}

{% set entriesCw32Ids = craft.entries.section('events').startDate(dateParamCw32).ids() %}
{% set entriesCw34Ids = craft.entries.section('events').startDate(dateParamCw34).ids() %}

{% set idParam = [] %}
{% set idParam = idParam|merge(entriesCw32Ids) %}
{% set idParam = idParam|merge(entriesCw34Ids) %}

{% set entries = craft.entries.id(idParam) %}