I agree with you that from an airmanship perspective, in a potentially marginal weather situation or whether you believe the GAF might not be a true reflection of the weather, it would be a very wise to carry an alternate.
However, this make no sense when considered from what a VFR aircraft can do.
"When an aerodrome forecast is not available or provisional, the PIC must make provision for a suitable alternate that has a firm forecast."
This from the AIP doesn't distinguish between IFR and VFR. If that was the case that if planning to an AD that doesnt have a TAF you must require an alternate, VFR aircraft wouldn't be able to fly to any old airport that doesn't have a regular forecast without an alternate, which as we known isn't what happens legally. If a VFR aircraft in a CAVOK day, as seen visually and from the the area forecast can fly to a non- TAF aerodrome without an alternate, why cant a IFR aircraft, both aircraft are in the exact same weather conditions ( CAVOK).
Secondly, the AIP quote about requiring an alternate for TAF unavailable airport begins with " For aerodromes with an Instrument Approach Procedure". Evidently, this rule doesnt apply to an aerodrome without an IAP as this rule is in relation to being able to determine alternate requirements based upon the alternate minima published on the IAL chart.
Thirdly, Why would the AIP publish the Alternate Minima for a non IAP destination to be " LSALT+ 5 and 8km for the last route segment". Nearly always, the last route segment wont be contained within the capture region of an Aerodrome forecast ( 5nm of ARP), thus the GAF comes into play.
The note about a forecaast being unaviable or provisional is under my understanding to be in regards to being outside a TAFs validity period ( thus making it unavailable for planning) or other factors that cause a normally issued TAF to not be valid/available .
In summary
ENR 1.1: For aerodromes without an IAP, the alternate minima is LSALT + 500ft for final route segement and 8km vis
ENR 1.10 For a flight for which a IAP does not exist, the minimum requirement is a GAF
Why would you require a TAF to avoid alternate if it states that for a non IAP destination, the minimum is a GAF ( for the sole reason that you don't need to compare Aerodrome weather with the alternate minima on the chart).
Since there is no IAP chart, you establish the final routes LSALT, add 500ft and consult the GAF, if there is more than SCT cloud below the minima you just established, you provide an alternate.
Why should a VFR aircraft be able to fly to a non - TAF destination without a alternate, as they have looked at the GAF ( which is the minimum ENR 1.10) and out the window and its blue skies everywhere, whilst a IFR aircraft must carry an alternate whilst flying in the exact same weather conditions...?
And to re-instate the non IAP destination requirements, by day you need not have an alternate if LSALT+5 and 8km vis for last route segment, ( as will be able to conduct a visual approach), but by night require an alternate regardless.
Dont get me wrong, if its very marginal weather outside or i doubt the GAFs accurarcy or by knowledge of local weather conditions prevailing, an alternate will be very good idea.
However, I dont agree or see how an IFR aircraft must have an alternate simply because there is no TAF, however a VFR aircraft looking at the same blue skies can happily plan to the same airport without alternate fuel.