Grantha script and it's associated scripts are not based on scientific analysis, rather they are based on hear-say additions. As the Grantha script was built on a random hear-say knowledge, it's ability to scientifically scale is very limited.
So expanding Grantha in a non scalable and non-structured manner to write certain Tamil passages is a very dangerous trend. As the random based script and it's associated scripts has a wide user base (population) this can lead to the dismantling of scientifically defined Tamil alphabet system, which is scalable and structured.
The scalable nature of the definition of Tamil alphabet, that of names of the places of articulation, gives rise to large number of phonemes in day to day usage of spoken Tamil. The number of phonemes used in Tamil is largest for any language in the world and the definition of alphabet in Tamil is based on scientific philosophy and different to any other language in the world.
Scientifically analysed, philosophy used in Tamil alphabet is the only correct approach. however, due to usage numeracy reason there is a danger that this foundation of all languages might be lost for ever by introducing random based system to replace Tamil methodology.
In another front, any attempt to expand the Tamil script base has to be done again on a scientific basis and on an scalable basis. Random, hear-say approaches should not be allowed to modify Tamil. (An example of expansion would be creating scalable diacritics to indicate the numerous phonemes for pronunciation dictionary. The diacritics must be defined structurally. Numerous vowel phonemes including half, full and in between vowels, all consonantal phonemes possible at of each of the places of articulations, definitions for (aytham) different types of modulations, such as aspirations need to be structurally defined. The theory of minimal number of alphabet to clearly describes all possible phonemes and modulations must be adhered to.)
In essence, using Grantha philosophy in Tamil would be a dangerous trend that can destroy the only surviving alphabet system that is scientifically accurate.