One of the reasons we design, install and service the Lochinvar Knight condensing boiler is the companies' liberal policy on the need for primary/secondary piping. Lochinvar will allow the use of direct piping to the distribution system as long as you provide the minimum flow required by all low-mass (not cast iron) heat exchangers. Like the use of anti-freeze, there is little to be gained beyond this the critical need to take the heat away from the heat exchanger and much to lose, such as the added cost of a redundant pump and the electricity is burns and as you astutely noted, the possibility of elevated return water temperature and resultant loss of potential combustion efficiency.
All condensing boilers now have differential temperature sensors that help monitor the critical minimum flow required and unlike the early condensing boilers will allow a higher differential without sustaining damage but the installer must assure this minimum by carefully calculating all potential flow rates. Multi-zone systems-especially those with small load or narrow long loops such as a radiant floor for the master bath for example-must be used with caution. We will often tie zones together and the call for heat only allowed on our minimum design load.
We also use differential by-pass valve on many systems. The DB is closed when all zones call allowing full flow through boiler and system and opens (by-passes back through the boiler) incrementally as zones are satisfied.
The people who sell condensing boilers should be helping a legitimate contractor with the near piping design before he starts the job.
We installed the first series of Knight boilers sporting the early and ubiquitous Ginanni "water-tube" boilers, which were much more restrictive than the new fire-tube models, but even so, rarely used primary/secondary piping. The WH-55 needs a modest minimum flow of 3gpm (found in the charts provided in the "Installation Manual" provided with every unit). With one-120' copper loop at 3gpm and approximately 1.5psi/3.5 fthd the up1558 will easily handle the load. The small condensing boilers tend to be easier to pipe for gas and water and as such very forgiving.
For others that may be watching, it pays to do the math before you hang a high efficiency condensing boiler on the wall.