VNA, it is generally regarded that there is not much advantage to be gained by having more than a 1000 hours of single engine VFR time. When JAA introducing the 500 multi crew time required for an ATPL then any single engine hours past 1000 will not help you achieve a green licence (frustratingly this also means single crew multi hours also do not count towards this requirement).
With regard to piston twin and light turbine twins, if it is single pilot ops then some airlines do view pilots with alot of single pilot time as a potential CRM problem, not sure what would constitute too much single ops time and not saying I necessarily agree with that view either.
While some airlines will view instructional hours as useful due to been able to progress you to command quicker, there are also some operators who will factor down single engine hours when determining time to command, I guess that is to stop an instructor with 2500 hours SEP time gaining a jet command within six months of employment, so in that sense the factoring down of single engine time is not unreasonable.